What is PL/SQL?
PL/SQL stands for Procedural Language extension of SQL.
PL/SQL is a combination of SQL along with the procedural features of programming languages. It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.
PL/SQL is a combination of SQL along with the procedural features of programming languages. It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.
The PL/SQL Engine:
Oracle uses a PL/SQL engine to processes the PL/SQL statements. A PL/SQL code can be stored in the client system (client-side) or in the database (server-side).
About This PL SQL
Programming tutorial
This Oracle PL SQL tutorial teaches you the basics of
programming in PL/SQL with appropriate examples. You can use this tutorial as
your guide or reference while programming with PL SQL. I will be making this
Oracle PL SQL programming tutorial as often as possible to share my knowledge
in PL SQL and help you in learning PL SQL better.
Even though the programming concepts discussed in this tutorial is specific to Oracle PL SQL. The concepts like cursors, functions and stored procedures can be used in other database systems like Sybase , Microsoft SQL server etc, with some change in syntax. This tutorial will be growing regularly; let us know if any topic related to PL SQL needs to be added or you can also share your knowledge on PL SQL with us. Lets share our knowledge about PL SQL with others.
Even though the programming concepts discussed in this tutorial is specific to Oracle PL SQL. The concepts like cursors, functions and stored procedures can be used in other database systems like Sybase , Microsoft SQL server etc, with some change in syntax. This tutorial will be growing regularly; let us know if any topic related to PL SQL needs to be added or you can also share your knowledge on PL SQL with us. Lets share our knowledge about PL SQL with others.
A Simple PL/SQL Block:
Each PL/SQL program consists of SQL and PL/SQL statements which
from a PL/SQL block.
A PL/SQL Block consists of three sections:
A PL/SQL Block consists of three sections:
- The
Declaration section (optional).
- The
Execution section (mandatory).
- The
Exception (or Error) Handling section (optional).
Declaration Section:
The Declaration section of a PL/SQL Block starts with the
reserved keyword DECLARE. This section is optional and is used to declare any
placeholders like variables, constants, records and cursors, which are used to
manipulate data in the execution section. Placeholders may be any of Variables,
Constants and Records, which stores data temporarily. Cursors are also declared
in this section.
Execution Section:
The Execution section of a PL/SQL Block starts with the reserved
keyword BEGIN and ends with END. This is a mandatory section and is the section
where the program logic is written to perform any task. The programmatic
constructs like loops, conditional statement and SQL statements form the part
of execution section.
Exception Section:
The Exception section of a PL/SQL Block starts with the reserved
keyword EXCEPTION. This section is optional. Any errors in the program can be
handled in this section, so that the PL/SQL Blocks terminates gracefully. If
the PL/SQL Block contains exceptions that cannot be handled, the Block
terminates abruptly with errors.
Every statement in the above three sections must end with a semicolon ; . PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.
This is how a sample PL/SQL Block looks.
Every statement in the above three sections must end with a semicolon ; . PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.
This is how a sample PL/SQL Block looks.
|
DECLARE
Variable declaration BEGIN Program Execution EXCEPTION Exception handling END; |
Advantages of PL/SQL
These are the advantages of PL/SQL.
- Block Structures: PL
SQL consists of blocks of code, which can be nested within each other.
Each block forms a unit of a task or a logical module. PL/SQL Blocks can
be stored in the database and reused.
- Procedural
Language Capability: PL
SQL consists of procedural language constructs such as conditional
statements (if else statements) and loops like (FOR loops).
- Better
Performance: PL
SQL engine processes multiple SQL statements simultaneously as a single
block, thereby reducing network traffic.
- Error Handling: PL/SQL
handles errors or exceptions effectively during the execution of a PL/SQL
program. Once an exception is caught, specific actions can be taken
depending upon the type of the exception or it can be displayed to the
user with a message.
PL/SQL Placeholders
Placeholders are
temporary storage area. Placeholders can be any of Variables, Constants and
Records. Oracle defines placeholders to store data temporarily, which are used
to manipulate data during the execution of a PL SQL block.
Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
Number (n,m) , Char (n) , Varchar2 (n) , Date , Long , Long raw, Raw, Blob, Clob, Nclob, Bfile
Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
Number (n,m) , Char (n) , Varchar2 (n) , Date , Long , Long raw, Raw, Blob, Clob, Nclob, Bfile
PL/SQL Variables
These are placeholders
that store the values that can change through the PL/SQL Block.
The General Syntax to declare a variable is:
The General Syntax to declare a variable is:
variable_name datatype
[NOT NULL := value ];
- variable_name is
the name of the variable.
- datatype is
a valid PL/SQL datatype.
- NOT
NULL is an optional specification on the variable.
- value or
DEFAULT valueis
also an optional specification, where you can initialize a variable.
- Each
variable declaration is a separate statement and must be terminated by a semicolon.
For example, if you want to store the current salary of an employee, you can use a variable.
DECLARE
salary number (6);
* “salary” is a variable of datatype number and of length 6.
When a variable is specified as NOT NULL, you must initialize the variable when it is declared.
For example: The below example declares two variables, one of which is a not null.
When a variable is specified as NOT NULL, you must initialize the variable when it is declared.
For example: The below example declares two variables, one of which is a not null.
DECLARE
salary number(4);
dept varchar2(10) NOT
NULL := “HR Dept”;
The value of a variable
can change in the execution or exception section of the PL/SQL Block. We can
assign values to variables in the two ways given below.
1) We can directly assign
values to variables.
The General Syntax is:
The General Syntax is:
variable_name:= value;
2) We can assign values to variables directly from the database
columns by using a SELECT.. INTO statement. The General Syntax is:
|
SELECT column_name
INTO variable_name
FROM table_name
[WHERE condition];
|
Example: The below
program will get the salary of an employee with id '1116' and display it on the
screen.
DECLARE
var_salary
number(6);
var_emp_id
number(6) = 1116;
BEGIN
SELECT salary
INTO var_salary
FROM employee
WHERE emp_id =
var_emp_id;
dbms_output.put_line(var_salary);
dbms_output.put_line('The
employee '
||
var_emp_id || ' has salary ' || var_salary);
END;
/
NOTE: The backward slash
'/' in the above program indicates to execute the above PL/SQL Block.
Scope
of Variables
PL/SQL allows the nesting
of Blocks within Blocks i.e, the Execution section of an outer block can contain
inner blocks. Therefore, a variable which is accessible to an outer Block is
also accessible to all nested inner Blocks. The variables declared in the inner
blocks are not accessible to outer blocks. Based on their declaration we can
classify variables into two types.
- Local variables
- These are declared in a inner block and cannot be referenced by outside
Blocks.
- Global variables
- These are declared in a outer block and can be referenced by its itself
and by its inner blocks.
For Example: In the below example we are creating two variables
in the outer block and assigning thier product to the third variable created in
the inner block. The variable 'var_mult' is declared in the inner block, so
cannot be accessed in the outer block i.e. it cannot be accessed after line 11.
The variables 'var_num1' and 'var_num2' can be accessed anywhere in the block.
|
1> DECLARE
2>
var_num1 number;
3> var_num2 number;
4> BEGIN
5> var_num1 := 100;
6> var_num2 := 200;
7> DECLARE
8> var_mult number;
9> BEGIN
10> var_mult :=
var_num1 * var_num2;
11> END;
12> END;
13> /
|
PL/SQL Constants
As the name implies a constant is a value used in a
PL/SQL Block that remains unchanged throughout the program. A constant is a
user-defined literal value. You can declare a constant and use it instead of
actual value.
For example: If you want
to write a program which will increase the salary of the employees by 25%, you
can declare a constant and use it throughout the program. Next time when you
want to increase the salary again you can change the value of the constant
which will be easier than changing the actual value throughout the program.
The General Syntax to declare a constant is:
|
constant_name CONSTANT datatype := VALUE;
|
- constant_name is
the name of the constant i.e. similar to a variable name.
- The
word CONSTANT is
a reserved word and ensures that the value does not change.
- VALUE -
It is a value which must be assigned to a constant when it is declared.
You cannot assign a value later.
For example, to declare
salary_increase, you can write code as follows:
DECLARE
salary_increase
CONSTANT number (3) := 10;
You must assign a value to a
constant at the time you declare it. If you do not assign a value to a constant
while declaring it and try to assign a value in the execution section, you will
get a error. If you execute the below Pl/SQL block you will get error.
|
DECLARE
salary_increase CONSTANT number(3);
BEGIN
salary_increase := 100;
dbms_output.put_line (salary_increase);
END;
|
PL/SQL Records
What are records?
Records are another type
of datatypes which oracle allows to be defined as a placeholder. Records are
composite datatypes, which means it is a combination of different scalar
datatypes like char, varchar, number etc. Each scalar data types in the
record holds a value. A record can be visualized as a row of data. It can
contain all the contents of a row.
Declaring a record:
To declare a record, you must first define a composite datatype;
then declare a record for that type.
The General Syntax to define a composite datatype is:
The General Syntax to define a composite datatype is:
|
TYPE record_type_name IS RECORD
(first_col_name column_datatype,
second_col_name column_datatype, ...);
|
- record_type_name – it is the name
of the composite type you want to define.
- first_col_name, second_col_name,
etc.,- it is the names the
fields/columns within the record.
- column_datatype defines the
scalar datatype of the fields.
There are different ways you can declare the datatype of the fields.
1) You can declare the
field in the same way as you declare the fieds while creating the table.
2) If a field is based on a column from database table, you can define the field_type as follows:
2) If a field is based on a column from database table, you can define the field_type as follows:
|
col_name table_name.column_name%type;
|
By declaring the field
datatype in the above method, the datatype of the column is dynamically applied
to the field. This method is useful when you are altering the column
specification of the table, because you do not need to change the code
again.
NOTE: You can use also %type to declare variables and constants.
The General Syntax to declare a record of a uer-defined datatype is:
The General Syntax to declare a record of a uer-defined datatype is:
|
record_name record_type_name;
|
The following code shows
how to declare a record called employee_rec based
on a user-defined type.
|
DECLARE
TYPE employee_type IS RECORD
(employee_id number(5),
employee_first_name varchar2(25),
employee_last_name
employee.last_name%type,
employee_dept employee.dept%type);
employee_salary employee.salary%type;
employee_rec employee_type;
|
If all the fields of a
record are based on the columns of a table, we can declare the record as
follows:
|
record_name table_name%ROWTYPE;
|
For example, the
above declaration of employee_rec can as follows:
|
DECLARE
employee_rec employee%ROWTYPE;
|
The advantages of declaring
the record as a ROWTYPE are:
1) You do not need to explicitly declare variables for all the columns in a table.
2) If you alter the column specification in the database table, you do not need to update the code.
1) You do not need to explicitly declare variables for all the columns in a table.
2) If you alter the column specification in the database table, you do not need to update the code.
The disadvantage of
declaring the record as a ROWTYPE is:
1) When u create a record as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields. So use ROWTYPE only when you are using all the columns of the table in the program.
1) When u create a record as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields. So use ROWTYPE only when you are using all the columns of the table in the program.
NOTE: When you are creating a record, you are just creating a
datatype, similar to creating a variable. You need to assign values to the
record to use them.
The following table consolidates the different ways in which you can define and declare a pl/sql record.
The following table consolidates the different ways in which you can define and declare a pl/sql record.
|
Syntax |
Usage
|
|
TYPE record_type_name IS RECORD (column_name1 datatype,
column_name2 datatype, ...);
|
Define a composite datatype, where each field is scalar.
|
|
col_name table_name.column_name%type;
|
Dynamically define the datatype of a column based on a
database column.
|
|
record_name record_type_name;
|
Declare a record based on a user-defined type.
|
|
record_name table_name%ROWTYPE;
|
Dynamically declare a record based on an entire row of a
table. Each column in the table corresponds to a field in the record.
|
Passing
Values To and From a Record
When you assign values to
a record, you actually assign values to the fields within it.
The General Syntax to assign a value to a column within a record direclty is:
The General Syntax to assign a value to a column within a record direclty is:
|
record_name.col_name := value;
|
If you used %ROWTYPE to
declare a record, you can assign values as shown:
|
record_name.column_name := value;
|
We can assign values to
records using SELECT Statements as shown:
|
SELECT col1, col2
INTO record_name.col_name1,
record_name.col_name2
FROM table_name
[WHERE clause];
|
If %ROWTYPE is used to
declare a record then you can directly assign values to the whole record
instead of each columns separately. In this case, you must SELECT all the
columns from the table into the record as shown:
|
SELECT * INTO record_name
FROM table_name
[WHERE clause];
|
Lets see how we can get
values from a record.
The General Syntax to retrieve a value from a specific field into another variable is:
The General Syntax to retrieve a value from a specific field into another variable is:
|
var_name := record_name.col_name;
|
The following table
consolidates the different ways you can assign values to and from a record:
|
Syntax
|
Usage
|
|
record_name.col_name := value;
|
To directly assign a value to a specific column of a record.
|
|
record_name.column_name := value;
|
To directly assign a value to a specific column of a record,
if the record is declared using %ROWTYPE.
|
|
SELECT col1, col2 INTO record_name.col_name1,
record_name.col_name2 FROM table_name [WHERE clause];
|
To assign values to each field of a record from the database
table.
|
|
SELECT * INTO record_name FROM table_name [WHERE clause];
|
To assign a value to all fields in the record from a database
table.
|
|
variable_name := record_name.col_name;
|
To get a value from a record column and assigning it to a
variable.
|
Conditional Statements in
PL/SQL
As the name implies, PL/SQL supports programming language features like conditional statements, iterative statements.
The programming constructs are similar to how you use in programming languages like Java and C++. In this section I will provide you syntax of how to use conditional statements in PL/SQL programming.
IF THEN ELSE STATEMENT
1)
IF condition
THEN
statement 1;
ELSE
statement 2;
END IF;
2)
IF condition 1
THEN
statement 1;
statement 2;
ELSIF condtion2 THEN
statement 3;
ELSE
statement 4;
END IF
3)
IF condition 1
THEN
statement 1;
statement 2;
ELSIF condtion2 THEN
statement 3;
ELSE
statement 4;
END IF;
4)
IF condition1 THEN
ELSE
IF condition2
THEN
statement1;
END IF;
ELSIF condition3 THEN
statement2;
END IF;
Iterative Statements in
PL/SQL
An iterative control Statements are used when we want to repeat the execution of one or more statements for specified number of times. These are similar to those in
There are three types of
loops in PL/SQL:
• Simple Loop
• While Loop
• For Loop
• Simple Loop
• While Loop
• For Loop
1) Simple Loop
A Simple Loop is used
when a set of statements is to be executed at least once before the loop terminates.
An EXIT condition must be specified in the loop, otherwise the loop will get
into an infinite number of iterations. When the EXIT condition is satisfied the
process exits from the loop.
The General Syntax to write a Simple Loop is:
LOOP
statements;
EXIT;
{or EXIT WHEN condition;}
END LOOP;
These are the important steps to be followed
while using Simple Loop.
1) Initialize a variable
before the loop body.
2) Increment the variable in the loop.
3) Use a EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once.
2) Increment the variable in the loop.
3) Use a EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once.
2) While Loop
A WHILE LOOP is used when
a set of statements has to be executed as long as a condition is true. The
condition is evaluated at the beginning of each iteration. The iteration
continues until the condition becomes false.
The General Syntax to write a WHILE LOOP is:
WHILE
<condition>
LOOP statements;
END LOOP;
Important steps to follow when executing a
while loop:
1) Initialize a variable
before the loop body.
2) Increment the variable in the loop.
3) EXIT WHEN statement and EXIT statements can be used in while loops but it's not done oftenly.
2) Increment the variable in the loop.
3) EXIT WHEN statement and EXIT statements can be used in while loops but it's not done oftenly.
3) FOR Loop
A FOR LOOP is used to
execute a set of statements for a predetermined number of times. Iteration
occurs between the start and end integer values given. The counter is always
incremented by 1. The loop exits when the counter reachs the value of the end
integer.
The General Syntax to
write a FOR LOOP is:
FOR counter IN
val1..val2
LOOP statements;
END LOOP;
- val1
- Start integer value.
- val2
- End integer value.
Important steps to follow when executing a
while loop:
1) The counter variable
is implicitly declared in the declaration section, so it's not necessary to
declare it explicity.
2) The counter variable is incremented by 1 and does not need to be incremented explicitly.
3) EXIT WHEN statement and EXIT statements can be used in FOR loops but it's not done oftenly.
2) The counter variable is incremented by 1 and does not need to be incremented explicitly.
3) EXIT WHEN statement and EXIT statements can be used in FOR loops but it's not done oftenly.
NOTE: The
above Loops are explained with a example when dealing with Explicit Cursors.
What are Cursors?
A cursor is a temporary
work area created in the system memory when a SQL statement is executed. A
cursor contains information on a select statement and the rows of data accessed
by it. This temporary work area is used to store the data retrieved from the database,
and manipulate this data. A cursor can hold more than one row, but can process
only one row at a time. The set of rows the cursor holds is called the active set.
There are two types of
cursors in PL/SQL:
Implicit
cursors:
These are created by
default when DML statements like, INSERT, UPDATE, and DELETE statements are
executed. They are also created when a SELECT statement that returns just one
row is executed.
Explicit
cursors:
They must be created
when you are executing a SELECT statement that returns more than one row. Even
though the cursor stores multiple records, only one record can be processed at
a time, which is called as current row. When you fetch a row the current row
position moves to next row.
Both implicit and
explicit cursors have the same functionality, but they differ in the way they
are accessed.
Implicit Cursors:
When you execute DML
statements like DELETE, INSERT, UPDATE and SELECT statements, implicit
statements are created to process these statements.
Oracle provides few attributes
called as implicit cursor attributes to check the status of DML operations. The
cursor attributes available are %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN.
For example, When you
execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us
whether any rows are affected and how many have been affected.
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.
The status of the cursor
for each of these attributes are defined in the below table.
|
Attributes
|
Return
Value
|
Example
|
|
%FOUND
|
The return value is TRUE, if the DML statements like INSERT,
DELETE and UPDATE affect at least one row and if SELECT ….INTO statement
return at least one row.
|
SQL%FOUND
|
|
The return value is FALSE, if DML statements like INSERT,
DELETE and UPDATE do not affect row and if SELECT….INTO statement do not
return a row.
|
||
|
%NOTFOUND
|
The return value is FALSE, if DML statements like INSERT,
DELETE and UPDATE at least one row and if SELECT ….INTO statement return at
least one row.
|
SQL%NOTFOUND
|
|
The return value is TRUE, if a DML statement like INSERT,
DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement
does not return a row.
|
||
|
%ROWCOUNT
|
Return the number of rows affected by the DML operations
INSERT, DELETE, UPDATE, SELECT
|
SQL%ROWCOUNT
|
For Example: Consider
the PL/SQL Block that uses implicit cursor attributes as shown below:
DECLARE var_rows number(5);
BEGIN
UPDATE employee
SET salary = salary + 1000;
IF SQL%NOTFOUND THEN
dbms_output.put_line('None of the salaries where updated');
ELSIF SQL%FOUND THEN
var_rows := SQL%ROWCOUNT;
dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');
END IF;
END;
In the above PL/SQL
Block, the salaries of all the employees in the ‘employee’ table are updated.
If none of the employee’s salary are updated we get a message 'None of the
salaries where updated'. Else we get a message like for example, 'Salaries for
1000 employees are updated' if there are 1000 rows in ‘employee’ table.
Explicit Cursors
An explicit cursor is
defined in the declaration section of the PL/SQL Block. It is created on a
SELECT Statement which returns more than one row. We can provide a suitable
name for the cursor.
The General Syntax for creating a cursor is as given below:
The General Syntax for creating a cursor is as given below:
CURSOR cursor_name IS select_statement;
- cursor_name
– A suitable name for the cursor.
- select_statement
– A select query which returns multiple rows.
How to use Explicit Cursor?
There are four steps in using an Explicit Cursor.
- DECLARE
the cursor in the declaration section.
- OPEN
the cursor in the Execution Section.
- FETCH
the data from cursor into PL/SQL variables or records in the Execution
Section.
- CLOSE
the cursor in the Execution Section before you end the PL/SQL Block.
1) Declaring a Cursor in
the Declaration Section:
DECLARE
CURSOR emp_cur IS
SELECT *
FROM emp_tbl
WHERE salary > 5000;
In
the above example we are creating a cursor ‘emp_cur’ on a query which returns
the records of all the employees
with salary greater than 5000. Here ‘emp_tbl’ in the table which contains
records of all the employees.
2) Accessing the records
in the cursor:
Once the cursor is created in the declaration section we can access the cursor in the execution
section of the PL/SQL program.
Once the cursor is created in the declaration section we can access the cursor in the execution
section of the PL/SQL program.
How to access an Explicit Cursor?
These are the three
steps in accessing the cursor.
1) Open the cursor.
2) Fetch the records in the cursor one at a time.
3) Close the cursor.
1) Open the cursor.
2) Fetch the records in the cursor one at a time.
3) Close the cursor.
General Syntax to open a
cursor is:
OPEN cursor_name;
General Syntax to fetch
records from a cursor is:
FETCH cursor_name INTO record_name;
OR
FETCH cursor_name INTO variable_list;
General Syntax to close
a cursor is:
CLOSE cursor_name;
When a cursor is opened,
the first row becomes the current row. When the data is fetched it is copied to
the record or variables and the logical pointer moves to the next row and it
becomes the current row. On every fetch statement, the pointer moves to the
next row. If you want to fetch after the last row, the program will throw an
error. When there is more than one row in a cursor we can use loops along with
explicit cursor attributes to fetch all the records.
Points to remember while
fetching a row:
· We can fetch the rows
in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL
Block.
· If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
· If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.
· If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
· If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.
General Form of using an
explicit cursor is:
DECLARE
variables;
records;
create a cursor;
BEGIN
OPEN cursor;
FETCH cursor;
process the records;
CLOSE cursor;
END;
Lets Look at the example
below
Example 1:
Example 1:
1> DECLARE
2> emp_rec emp_tbl%rowtype;
3> CURSOR emp_cur IS
4> SELECT *
5> FROM
6> WHERE salary > 10;
7> BEGIN
8> OPEN emp_cur;
9> FETCH emp_cur INTO emp_rec;
10> dbms_output.put_line (emp_rec.first_name || ' ' || emp_rec.last_name);
11> CLOSE emp_cur;
12> END;
In the above example,
first we are creating a record ‘emp_rec’ of the same structure as of table
‘emp_tbl’ in line no 2. We can also create a record with a cursor by replacing
the table name with the cursor name. Second, we are declaring a cursor
‘emp_cur’ from a select query in line no 3 - 6. Third, we are opening the
cursor in the execution section in line no 8. Fourth, we are fetching the
cursor to the record in line no 9. Fifth, we are displaying the first_name and
last_name of the employee in the record emp_rec in line no 10. Sixth, we are
closing the cursor in line no 11.
What are Explicit Cursor Attributes?
Oracle provides some
attributes known as Explicit Cursor Attributes to control the data processing
while using cursors. We use these attributes to avoid errors while accessing
cursors through OPEN, FETCH and CLOSE Statements.
When does an error occur while accessing an explicit
cursor?
a) When we try to open a
cursor which is not closed in the previous operation.
b) When we try to fetch a cursor after the last operation.
b) When we try to fetch a cursor after the last operation.
These are the attributes
available to check the status of an explicit cursor.
|
Attributes
|
Return
values
|
Example
|
|
%FOUND
|
TRUE, if fetch statement returns
at least one row.
|
Cursor_name%FOUND
|
|
FALSE, if fetch statement doesn’t
return a row.
|
||
|
%NOTFOUND
|
TRUE, , if fetch statement doesn’t
return a row.
|
Cursor_name%NOTFOUND
|
|
FALSE, if fetch statement returns
at least one row.
|
||
|
%ROWCOUNT
|
The number of rows fetched by the
fetch statement
|
Cursor_name%ROWCOUNT
|
|
If no row is returned, the PL/SQL
statement returns an error.
|
||
|
%ISOPEN
|
TRUE, if the cursor is already
open in the program
|
Cursor_name%ISNAME
|
|
FALSE, if the cursor is not opened
in the program.
|
Using Loops with Explicit Cursors:
Oracle provides three
types of cursors namely SIMPLE LOOP, WHILE LOOP and FOR LOOP. These loops can
be used to process multiple rows in the cursor. Here I will modify the same
example for each loops to explain how to use loops with cursors.
Cursor with a Simple
Loop:
1> DECLARE
2> CURSOR emp_cur IS
3> SELECT first_name, last_name, salary FROM emp_tbl;
4> emp_rec emp_cur%rowtype;
5> BEGIN
6> IF NOT sales_cur%ISOPEN THEN
7> OPEN sales_cur;
8> END IF;
9> LOOP
10> FETCH emp_cur INTO emp_rec;
11> EXIT WHEN emp_cur%NOTFOUND;
12> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
13> || ' ' ||emp_cur.salary);
14> END LOOP;
15> END;
16> /
In the above example we
are using two cursor attributes %ISOPEN and %NOTFOUND.
In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9.
In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues.
In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9.
In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues.
We can use %FOUND in
place of %NOTFOUND and vice versa. If we do so, we need to reverse the logic of
the program. So use these attributes in appropriate instances.
Cursor with a While
Loop:
Lets modify the above
program to use while loop.
1> DECLARE
2> CURSOR emp_cur IS
3> SELECT first_name, last_name, salary FROM emp_tbl;
4> emp_rec emp_cur%rowtype;
5> BEGIN
6> IF NOT sales_cur%ISOPEN THEN
7> OPEN sales_cur;
8> END IF;
9> FETCH sales_cur INTO sales_rec;
10> WHILE sales_cur%FOUND THEN
11> LOOP
12> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
13> || ' ' ||emp_cur.salary);
15> FETCH sales_cur INTO sales_rec;
16> END LOOP;
17> END;
18> /
In the above example, in
line no 10 we are using %FOUND to evaluate if the first fetch statement in line
no 9 returned a row, if true the program moves into the while loop. In the loop
we use fetch statement again (line no 15) to process the next row. If the fetch
statement is not executed once before the while loop the while condition will
return false in the first instance and the while loop is skipped. In the loop,
before fetching the record again, always process the record retrieved by the
first fetch statement, else you will skip the first row.
Cursor with a FOR Loop:
When using FOR LOOP you
need not declare a record or variables to store the cursor values, need not
open, fetch and close the cursor. These functions are accomplished by the FOR
LOOP automatically.
General Syntax
for using FOR LOOP:
FOR record_name IN cusror_name
LOOP
process the row...
END LOOP;
Let’s use the above
example to learn how to use for loops in cursors.
1> DECLARE
2> CURSOR emp_cur IS
3> SELECT first_name, last_name, salary FROM emp_tbl;
4> emp_rec emp_cur%rowtype;
5> BEGIN
6> FOR emp_rec in sales_cur
7> LOOP
8> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
9> || ' ' ||emp_cur.salary);
10> END LOOP;
11>END;
12> /
In the above example,
when the FOR loop is processed a record ‘emp_rec’of structure ‘emp_cur’ gets
created, the cursor is opened, the rows are fetched to the record ‘emp_rec’ and
the cursor is closed after the last row is processed. By using FOR Loop in your
program, you can reduce the number of lines in the program.
NOTE: In the examples given above, we are
using backward slash ‘/’ at the end of the program. This indicates the oracle
engine that the PL/SQL program has ended and it can begin processing the
statements.
Stored Procedures
What is a Stored
Procedure?
A stored procedure or in
simple a proc is a
named PL/SQL block which performs one or more specific task. This is similar to
a procedure in other programming languages. A procedure has a header and a
body. The header consists of the name of the procedure and the parameters or
variables passed to the procedure. The body consists or declaration section,
execution section and exception section similar to a general PL/SQL Block. A
procedure is similar to an anonymous PL/SQL Block but it is named for repeated
usage.
We can pass parameters to
procedures in three ways.
1) IN-parameters
2) OUT-parameters
3) IN OUT-parameters
1) IN-parameters
2) OUT-parameters
3) IN OUT-parameters
A
procedure may or may not return any value.
General Syntax to create
a procedure is:
CREATE [OR REPLACE]
PROCEDURE proc_name [list of parameters]
IS
Declaration section
BEGIN
Execution section
EXCEPTION
Exception section
END;
IS - marks the beginning of the body of the procedure and is similar
to DECLARE in anonymous PL/SQL Blocks. The code between IS and BEGIN forms the
Declaration section.
The syntax within the
brackets [ ] indicate they are optional. By using CREATE OR REPLACE together
the procedure is created if no other procedure with the same name exists or the
existing procedure is replaced with the current code.
The below example creates
a procedure ‘employer_details’ which gives the details of the employee.
1> CREATE OR
REPLACE PROCEDURE employer_details
2> IS
3> CURSOR emp_cur IS
4> SELECT first_name, last_name, salary FROM
emp_tbl;
5> emp_rec emp_cur%rowtype;
6> BEGIN
7> FOR emp_rec in sales_cur
8> LOOP
9> dbms_output.put_line(emp_cur.first_name || '
' ||emp_cur.last_name
10> || ' ' ||emp_cur.salary);
11> END LOOP;
12>END;
13> /
How to execute a Stored
Procedure?
There are two ways to
execute a procedure.
1) From the SQL prompt.
EXECUTE [or EXEC] procedure_name;
2) Within another
procedure – simply use the procedure name.
procedure_name;
NOTE: In the examples given above, we are using backward slash ‘/’ at
the end of the program. This indicates the oracle engine that the PL/SQL
program has ended and it can begin processing the statements.
PL/SQL Functions
What is a Function in
PL/SQL?
A function is a named
PL/SQL Block which is similar to a procedure. The major difference between a
procedure and a function is, a function
must always return a value, but a procedure may or may not return a value.
The General Syntax to
create a function is:
CREATE [OR REPLACE]
FUNCTION function_name [parameters]
RETURN
return_datatype;
IS
Declaration_section
BEGIN
Execution_section
Return
return_variable;
EXCEPTION
exception section
Return
return_variable;
END;
1) Return Type: The
header section defines the return type of the function. The return datatype can
be any of the oracle datatype like varchar, number etc.
2) The execution and exception section both should return a value which is of the datatype defined in the header section.
2) The execution and exception section both should return a value which is of the datatype defined in the header section.
For example, let’s create
a frunction called ''employer_details_func' similar to the one created in
stored proc
1> CREATE OR
REPLACE FUNCTION employer_details_func
2> RETURN VARCHAR(20);
3> IS
5> emp_name VARCHAR(20);
6> BEGIN
7> SELECT first_name INTO emp_name
8> FROM emp_tbl WHERE empID = '100';
9> RETURN emp_name;
10> END;
11> /
In the example we are
retrieving the ‘first_name’ of employee with empID 100 to variable ‘emp_name’.
The return type of the function is VARCHAR which is declared in line no 2.
The function returns the 'emp_name' which is of type VARCHAR as the return value in line no 9.
The return type of the function is VARCHAR which is declared in line no 2.
The function returns the 'emp_name' which is of type VARCHAR as the return value in line no 9.
How to execute a PL/SQL
Function?
A function can be
executed in the following ways.
1) Since a function
returns a value we can assign it to a variable.
employee_name := employer_details_func;
If ‘employee_name’ is of
datatype varchar we can store the name of the employee by assigning the return
type of the function to it.
2) As a part of a SELECT
statement
SELECT
employer_details_func FROM dual;
3) In a PL/SQL Statements
like,
dbms_output.put_line(employer_details_func);
This
line displays the value returned by the function.
Parameters in Procedure
and Functions
How to pass parameters to
Procedures and Functions in PL/SQL ?
In PL/SQL, we can pass
parameters to procedures and functions in three ways.
1) IN
type parameter: These
types of parameters are used to send values to stored procedures.
2) OUT type parameter: These types of parameters are used to get values from stored procedures. This is similar to a return type in functions.
3) IN OUT parameter: These types of parameters are used to send values and get values from stored procedures.
2) OUT type parameter: These types of parameters are used to get values from stored procedures. This is similar to a return type in functions.
3) IN OUT parameter: These types of parameters are used to send values and get values from stored procedures.
NOTE: If a
parameter is not explicitly defined a parameter type, then by default it is an
IN type parameter.
1) IN parameter:
This is similar to
passing parameters in programming languages. We can pass values to the stored
procedure through these parameters or variables. This type of parameter is a
read only parameter. We can assign the value of IN type parameter to a variable
or use it in a query, but we cannot change its value inside the procedure.
The General syntax to
pass a IN parameter is
CREATE [OR REPLACE]
PROCEDURE procedure_name (
param_name1 IN datatype, param_name12 IN
datatype ... )
- param_name1,
param_name2... are unique parameter names.
- datatype
- defines the datatype of the variable.
- IN
- is optional, by default it is a IN type parameter.
2) OUT Parameter:
The OUT parameters are
used to send the OUTPUT from a procedure or a function. This is a write-only
parameter i.e, we cannot pass values to OUT paramters while executing the
stored procedure, but we can assign values to OUT parameter inside the stored
procedure and the calling program can recieve this output value.
The General syntax to
create an OUT parameter is
CREATE [OR REPLACE]
PROCEDURE proc2 (param_name OUT datatype)
The parameter should be
explicity declared as OUT parameter.
3) IN OUT Parameter:
3) IN OUT Parameter:
The IN OUT parameter
allows us to pass values into a procedure and get output values from the
procedure. This parameter is used if the value of the IN parameter can be
changed in the calling program.
By using IN OUT parameter
we can pass values into a parameter and return a value to the calling program
using the same parameter. But this is possible only if the value passed to the
procedure and output value have a same datatype. This parameter is used if the
value of the parameter will be changed in the procedure.
The General syntax to
create an IN OUT parameter is
CREATE [OR REPLACE]
PROCEDURE proc3 (param_name IN OUT datatype)
The below examples show
how to create stored procedures using the above three types of parameters.
Example1:
Using
IN and OUT parameter:
Let’s create a procedure
which gets the name of the employee when the employee id is passed.
1> CREATE OR
REPLACE PROCEDURE emp_name (id IN NUMBER, emp_name OUT NUMBER)
2> IS
3> BEGIN
4> SELECT first_name INTO emp_name
5> FROM emp_tbl WHERE empID = id;
6> END;
7> /
We can call the procedure
‘emp_name’ in this way from a PL/SQL Block.
1> DECLARE
2> empName varchar(20);
3> CURSOR id_cur SELECT id FROM emp_ids;
4> BEGIN
5> FOR emp_rec in
id_cur
6> LOOP
7> emp_name(emp_rec.id, empName);
8> dbms_output.putline('The employee ' ||
empName || ' has id ' || emp-rec.id);
9> END LOOP;
10> END;
11> /
In the above PL/SQL Block
In line no 3; we are creating a cursor ‘id_cur’ which contains the employee id.
In line no 7; we are calling the procedure ‘emp_name’, we are passing the ‘id’ as IN parameter and ‘empName’ as OUT parameter.
In line no 8; we are displaying the id and the employee name which we got from the procedure ‘emp_name’.
In line no 3; we are creating a cursor ‘id_cur’ which contains the employee id.
In line no 7; we are calling the procedure ‘emp_name’, we are passing the ‘id’ as IN parameter and ‘empName’ as OUT parameter.
In line no 8; we are displaying the id and the employee name which we got from the procedure ‘emp_name’.
Example 2:
Using
IN OUT parameter in procedures:
1> CREATE OR
REPLACE PROCEDURE emp_salary_increase
2> (emp_id IN
emptbl.empID%type, salary_inc IN OUT emptbl.salary%type)
3> IS
4> tmp_sal number;
5> BEGIN
6> SELECT salary
7> INTO tmp_sal
8> FROM emp_tbl
9> WHERE empID = emp_id;
10> IF tmp_sal between 10000 and 20000 THEN
11> salary_inout := tmp_sal * 1.2;
12> ELSIF tmp_sal between 20000 and 30000 THEN
13> salary_inout := tmp_sal * 1.3;
14> ELSIF tmp_sal > 30000 THEN
15> salary_inout := tmp_sal * 1.4;
16> END IF;
17> END;
18> /
The below PL/SQL block
shows how to execute the above 'emp_salary_increase' procedure.
1> DECLARE
2> CURSOR updated_sal is
3> SELECT empID,salary
4> FROM emp_tbl;
5> pre_sal number;
6> BEGIN
7> FOR emp_rec IN updated_sal LOOP
8> pre_sal := emp_rec.salary;
9> emp_salary_increase(emp_rec.empID,
emp_rec.salary);
10> dbms_output.put_line('The salary of ' ||
emp_rec.empID ||
11> ' increased from '|| pre_sal || '
to '||emp_rec.salary);
12> END LOOP;
13> END;
14> /
Exception Handling
In this section we will
discuss about the following,
1) What is Exception Handling.
2) Structure of Exception Handling.
3) Types of Exception Handling.
1) What is Exception Handling.
2) Structure of Exception Handling.
3) Types of Exception Handling.
1) What is Exception
Handling?
PL/SQL provides a feature
to handle the Exceptions which occur in a PL/SQL Block known as exception
Handling. Using Exception Handling we can test the code and avoid it from
exiting abruptly. When an exception occurs a messages which explains its cause
is recieved.
PL/SQL Exception message consists of three parts.
1) Type of Exception
2) An Error Code
3) A message
By Handling the exceptions we can ensure a PL/SQL block does not exit abruptly.
PL/SQL Exception message consists of three parts.
1) Type of Exception
2) An Error Code
3) A message
By Handling the exceptions we can ensure a PL/SQL block does not exit abruptly.
2) Structure of Exception
Handling.
The General Syntax for
coding the exception section
DECLARE
Declaration section
BEGIN
Exception section
EXCEPTION
WHEN ex_name1 THEN
-Error handling statements
WHEN ex_name2 THEN
-Error handling statements
WHEN Others THEN
-Error handling statements
END;
General PL/SQL statments
can be used in the Exception Block.
When an exception is
raised, Oracle searches for an appropriate exception handler in the exception
section. For example in the above example, if the error raised is 'ex_name1 ',
then the error is handled according to the statements under it. Since, it is
not possible to determine all the possible runtime errors during testing fo the
code, the 'WHEN Others' exception is used to manage the exceptions that are not
explicitly handled. Only one exception can be raised in a Block and the control
does not return to the Execution Section after the error is handled.
If there are nested
PL/SQL blocks like this.
DELCARE
Declaration section
BEGIN
DECLARE
Declaration section
BEGIN
Execution section
EXCEPTION
Exception section
END;
EXCEPTION
Exception section
END;
In the above case, if the
exception is raised in the inner block it should be handled in the exception
block of the inner PL/SQL block else the control moves to the Exception block
of the next upper PL/SQL Block. If none of the blocks handle the exception the
program ends abruptly with an error.
3) Types of Exception.
There are 3 types of
Exceptions.
a) Named System Exceptions
b) Unnamed System Exceptions
c) User-defined Exceptions
a) Named System Exceptions
b) Unnamed System Exceptions
c) User-defined Exceptions
a) Named System
Exceptions
System exceptions are
automatically raised by Oracle, when a program violates a RDBMS rule. There are
some system exceptions which are raised frequently, so they are pre-defined and
given a name in Oracle which are known as Named System Exceptions.
For
example: NO_DATA_FOUND
and ZERO_DIVIDE are called Named System exceptions.
Named system exceptions
are:
1) Not Declared explicitly,
2) Raised implicitly when a predefined Oracle error occurs,
3) caught by referencing the standard name within an exception-handling routine.
1) Not Declared explicitly,
2) Raised implicitly when a predefined Oracle error occurs,
3) caught by referencing the standard name within an exception-handling routine.
|
Exception
Name
|
Reason
|
Error
Number
|
|
CURSOR_ALREADY_OPEN
|
When you open a cursor that is already open.
|
ORA-06511
|
|
INVALID_CURSOR
|
When you perform an invalid operation on a cursor like closing
a cursor, fetch data from a cursor that is not opened.
|
ORA-01001
|
|
NO_DATA_FOUND
|
When a SELECT...INTO clause does not return any row from a
table.
|
ORA-01403
|
|
TOO_MANY_ROWS
|
When you SELECT or fetch more than one row into a record or variable.
|
ORA-01422
|
|
ZERO_DIVIDE
|
When you attempt to divide a number by zero.
|
ORA-01476
|
For
Example: Suppose a NO_DATA_FOUND exception is raised in a proc, we can
write a code to handle the exception as given below.
BEGIN
Execution section
EXCEPTION
WHEN NO_DATA_FOUND
THEN
dbms_output.put_line ('A SELECT...INTO did not
return any row.');
END;
b) Unnamed System
Exceptions
Those system exception
for which oracle does not provide a name is known as unamed system exception.
These exception do not occur frequently. These Exceptions have a code and an
associated message.
There are two ways to
handle unnamed sysyem exceptions:
1. By using the WHEN OTHERS exception handler, or
2. By associating the exception code to a name and using it as a named exception.
1. By using the WHEN OTHERS exception handler, or
2. By associating the exception code to a name and using it as a named exception.
We can assign a name to
unnamed system exceptions using a Pragma called EXCEPTION_INIT.
EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.
EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.
Steps to be followed to
use unnamed system exceptions are
• They are raised implicitly.
• If they are not handled in WHEN Others they must be handled explicity.
• To handle the exception explicity, they must be declared using Pragma EXCEPTION_INIT as given above and handled referecing the user-defined exception name in the exception section.
• They are raised implicitly.
• If they are not handled in WHEN Others they must be handled explicity.
• To handle the exception explicity, they must be declared using Pragma EXCEPTION_INIT as given above and handled referecing the user-defined exception name in the exception section.
The general syntax to
declare unnamed system exception using EXCEPTION_INIT is:
DECLARE
exception_name EXCEPTION;
PRAGMA
EXCEPTION_INIT (exception_name, Err_code);
BEGIN
Execution section
EXCEPTION
WHEN exception_name THEN
handle the exception
END;
For
Example: Lets
consider the product table and order_items table from sql joins.
Here product_id is a
primary key in product table and a foreign key in order_items table.
If we try to delete a product_id from the product table when it has child records in order_id table an exception will be thrown with oracle code number -2292.
We can provide a name to this exception and handle it in the exception section as given below.
If we try to delete a product_id from the product table when it has child records in order_id table an exception will be thrown with oracle code number -2292.
We can provide a name to this exception and handle it in the exception section as given below.
DECLARE
Child_rec_exception EXCEPTION;
PRAGMA
EXCEPTION_INIT (Child_rec_exception, -2292);
BEGIN
Delete FROM product where product_id= 104;
EXCEPTION
WHEN Child_rec_exception
THEN Dbms_output.put_line('Child records are
present for this product_id.');
END;
/
c) User-defined
Exceptions
Apart from sytem
exceptions we can explicity define exceptions based on business rules. These
are known as user-defined exceptions.
Steps to be followed to
use user-defined exceptions:
• They should be explicitly declared in the declaration section.
• They should be explicitly raised in the Execution Section.
• They should be handled by referencing the user-defined exception name in the exception section.
• They should be explicitly declared in the declaration section.
• They should be explicitly raised in the Execution Section.
• They should be handled by referencing the user-defined exception name in the exception section.
For
Example: Lets
consider the product table and order_items table from sql joins to explain
user-defined exception.
Lets create a business rule that if the total no of units of any particular product sold is more than 20, then it is a huge quantity and a special discount should be provided.
Lets create a business rule that if the total no of units of any particular product sold is more than 20, then it is a huge quantity and a special discount should be provided.
DECLARE
huge_quantity EXCEPTION;
CURSOR product_quantity is
SELECT p.product_name as name,
sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
quantity order_tems.total_units%type;
up_limit CONSTANT order_tems.total_units%type
:= 20;
message VARCHAR2(50);
BEGIN
FOR product_rec in product_quantity LOOP
quantity := product_rec.units;
IF quantity > up_limit THEN
message := 'The number of units of
product ' || product_rec.name ||
' is more than 20. Special
discounts should be provided.
Rest of the records are skipped. '
RAISE huge_quantity;
ELSIF quantity < up_limit THEN
v_message:= 'The number of unit is below
the discount limit.';
END IF;
dbms_output.put_line (message);
END LOOP;
EXCEPTION
WHEN huge_quantity THEN
dbms_output.put_line (message);
END;
/
RAISE_APPLICATION_ERROR (
)
RAISE_APPLICATION_ERROR is a built-in procedure in oracle which is used to display the
user-defined error messages along with the error number whose range is in
between -20000 and -20999.
Whenever a message is
displayed using RAISE_APPLICATION_ERROR, all previous transactions which are
not committed within the PL/SQL Block are rolled back automatically (i.e.
change due to INSERT, UPDATE, or DELETE statements).
RAISE_APPLICATION_ERROR
raises an exception but does not handle it.
RAISE_APPLICATION_ERROR
is used for the following reasons,
a) to create a unique id for an user-defined exception.
b) to make the user-defined exception look like an Oracle error.
a) to create a unique id for an user-defined exception.
b) to make the user-defined exception look like an Oracle error.
The General Syntax to use
this procedure is:
RAISE_APPLICATION_ERROR
(error_number, error_message);
• The Error number must be between -20000 and -20999
• The Error_message is the message you want to display when the error occurs.
Steps to be folowed to
use RAISE_APPLICATION_ERROR procedure:
1. Declare a user-defined exception in the declaration section.
2. Raise the user-defined exception based on a specific business rule in the execution section.
3. Finally, catch the exception and link the exception to a user-defined error number in RAISE_APPLICATION_ERROR.
1. Declare a user-defined exception in the declaration section.
2. Raise the user-defined exception based on a specific business rule in the execution section.
3. Finally, catch the exception and link the exception to a user-defined error number in RAISE_APPLICATION_ERROR.
Using the above example
we can display a error message using RAISE_APPLICATION_ERROR.
DECLARE
huge_quantity EXCEPTION;
CURSOR product_quantity is
SELECT p.product_name as name,
sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
quantity order_tems.total_units%type;
up_limit CONSTANT order_tems.total_units%type
:= 20;
message VARCHAR2(50);
BEGIN
FOR product_rec in product_quantity LOOP
quantity := product_rec.units;
IF quantity > up_limit THEN
RAISE huge_quantity;
ELSIF quantity < up_limit THEN
v_message:= 'The number of unit is below
the discount limit.';
END IF;
Dbms_output.put_line (message);
END LOOP;
EXCEPTION
WHEN huge_quantity THEN
raise_application_error(-2100, 'The
number of unit is above the discount limit.');
END;
/
What is a Trigger?
A trigger is a pl/sql
block structure which is fired when a DML statements like Insert, Delete,
Update is executed on a database table. A trigger is triggered automatically
when an associated DML statement is executed.
Syntax of Triggers
The Syntax for creating a trigger is:
CREATE [OR REPLACE ] TRIGGER trigger_name
{BEFORE | AFTER | INSTEAD OF }
{INSERT [OR] | UPDATE [OR] | DELETE}
[OF col_name]
ON table_name
[REFERENCING OLD AS o NEW AS n]
[FOR EACH ROW]
WHEN (condition)
BEGIN
--- sql statements
END;
- CREATE [OR
REPLACE ] TRIGGER trigger_name - This clause
creates a trigger with the given name or overwrites an existing trigger
with the same name.
- {BEFORE | AFTER
| INSTEAD OF } - This clause indicates at what
time should the trigger get fired. i.e for example: before or after
updating a table. INSTEAD OF is used to create a trigger on a view. before
and after cannot be used to create a trigger on a view.
- {INSERT [OR] |
UPDATE [OR] | DELETE} -
This clause determines the triggering event. More than one triggering
events can be used together separated by OR keyword. The trigger gets
fired at all the specified triggering event.
- [OF
col_name] -
This clause is used with update triggers. This clause is used when you
want to trigger an event only when a specific column is updated.
- CREATE [OR
REPLACE ] TRIGGER trigger_name - This clause
creates a trigger with the given name or overwrites an existing trigger
with the same name.
- [ON
table_name] - This clause identifies the name of
the table or view to which the trigger is associated.
- [REFERENCING OLD
AS o NEW AS n] - This clause is used to reference
the old and new values of the data being changed. By default, you
reference the values as :old.column_name or :new.column_name. The reference
names can also be changed from old (or new) to any other user-defined
name. You cannot reference old values when inserting a record, or new
values when deleting a record, because they do not exist.
- [FOR EACH
ROW] -
This clause is used to determine whether a trigger must fire when each row
gets affected ( i.e. a Row Level Trigger) or just once when the entire sql
statement is executed(i.e.statement level Trigger).
- WHEN
(condition) - This clause is valid only for
row level triggers. The trigger is fired only for rows that satisfy the
condition specified.
For
Example: The
price of a product changes constantly. It is important to maintain the history
of the prices of the products.
We can create a trigger
to update the 'product_price_history' table when the price of the product is
updated in the 'product' table.
1) Create the 'product' table and 'product_price_history' table
CREATE TABLE
product_price_history
(product_id number(5),
product_name
varchar2(32),
supplier_name
varchar2(32),
unit_price number(7,2)
);
CREATE TABLE product
(product_id number(5),
product_name
varchar2(32),
supplier_name
varchar2(32),
unit_price number(7,2)
);
2) Create the price_history_trigger and execute it.
CREATE or REPLACE
TRIGGER price_history_trigger
BEFORE UPDATE OF
unit_price
ON product
FOR EACH ROW
BEGIN
INSERT INTO
product_price_history
VALUES
(:old.product_id,
:old.product_name,
:old.supplier_name,
:old.unit_price);
END;
/
3) Lets update the price of a product.
UPDATE PRODUCT SET
unit_price = 800 WHERE product_id = 100
Once the above update
query is executed, the trigger fires and updates the 'product_price_history'
table.
4)If you
ROLLBACK the transaction before committing to the database, the data inserted
to the table is also rolled back.
Types of PL/SQL Triggers
There are two types of
triggers based on the which level it is triggered.
1) Row level trigger - An event is triggered for each row upated, inserted or deleted.
2) Statement level trigger - An event is triggered for each sql statement executed.
1) Row level trigger - An event is triggered for each row upated, inserted or deleted.
2) Statement level trigger - An event is triggered for each sql statement executed.
PL/SQL Trigger Execution
Hierarchy
The following hierarchy
is followed when a trigger is fired.
1) BEFORE statement trigger fires first.
2) Next BEFORE row level trigger fires, once for each row affected.
3) Then AFTER row level trigger fires once for each affected row. This events will alternates between BEFORE and AFTER row level triggers.
4) Finally the AFTER statement level trigger fires.
1) BEFORE statement trigger fires first.
2) Next BEFORE row level trigger fires, once for each row affected.
3) Then AFTER row level trigger fires once for each affected row. This events will alternates between BEFORE and AFTER row level triggers.
4) Finally the AFTER statement level trigger fires.
For
Example: Let's
create a table 'product_check' which we can use to store messages when triggers
are fired.
CREATE TABLE product
(Message varchar2(50),
Current_Date number(32)
);
Let's create a BEFORE and
AFTER statement and row level triggers for the product table.
1)
BEFORE UPDATE, Statement Level: This
trigger will insert a record into the table 'product_check' before a sql update
statement is executed, at the statement level.
CREATE or REPLACE
TRIGGER Before_Update_Stat_product
BEFORE
UPDATE ON product
Begin
INSERT INTO
product_check
Values('Before update,
statement level',sysdate);
END;
/
2)
BEFORE UPDATE, Row Level: This trigger will insert
a record into the table 'product_check' before each row is updated.
CREATE or REPLACE TRIGGER
Before_Upddate_Row_product
BEFORE
UPDATE ON product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('Before update row level',sysdate);
END;
/
3)
AFTER UPDATE, Statement Level: This
trigger will insert a record into the table 'product_check' after a sql update
statement is executed, at the statement level.
CREATE or REPLACE TRIGGER
After_Update_Stat_product
AFTER
UPDATE ON product
BEGIN
INSERT INTO product_check
Values('After update, statement level',
sysdate);
End;
/
4)
AFTER UPDATE, Row Level: This trigger will insert
a record into the table 'product_check' after each row is updated.
CREATE or REPLACE TRIGGER
After_Update_Row_product
AFTER
insert On product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('After update, Row level',sysdate);
END;
/
Now lets execute a update
statement on table product.
UPDATE PRODUCT SET unit_price = 800
WHERE product_id in (100,101);
Lets check the data in
'product_check' table to see the order in which the trigger is fired.
SELECT * FROM product_check;
Output:
Mesage
Current_Date
------------------------------------------------------------
Before update, statement
level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
After update, statement level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
After update, statement level 26-Nov-2008
The above result shows
'before update' and 'after update' row level events have occured twice, since
two records were updated. But 'before update' and 'after update' statement
level events are fired only once per sql statement.
The above rules apply
similarly for INSERT and DELETE statements.
How To know Information
about Triggers.
We can use the data
dictionary view 'USER_TRIGGERS' to obtain information about any trigger.
The below statement shows
the structure of the view 'USER_TRIGGERS'
DESC USER_TRIGGERS;
NAME
Type
--------------------------------------------------------
TRIGGER_NAME
VARCHAR2(30)
TRIGGER_TYPE VARCHAR2(16)
TRIGGER_EVENT VARCHAR2(75)
TABLE_OWNER VARCHAR2(30)
BASE_OBJECT_TYPE VARCHAR2(16)
TABLE_NAME VARCHAR2(30)
COLUMN_NAME VARCHAR2(4000)
REFERENCING_NAMES VARCHAR2(128)
WHEN_CLAUSE VARCHAR2(4000)
STATUS VARCHAR2(8)
DESCRIPTION VARCHAR2(4000)
ACTION_TYPE VARCHAR2(11)
TRIGGER_BODY LONG
TRIGGER_TYPE VARCHAR2(16)
TRIGGER_EVENT VARCHAR2(75)
TABLE_OWNER VARCHAR2(30)
BASE_OBJECT_TYPE VARCHAR2(16)
TABLE_NAME VARCHAR2(30)
COLUMN_NAME VARCHAR2(4000)
REFERENCING_NAMES VARCHAR2(128)
WHEN_CLAUSE VARCHAR2(4000)
STATUS VARCHAR2(8)
DESCRIPTION VARCHAR2(4000)
ACTION_TYPE VARCHAR2(11)
TRIGGER_BODY LONG
This view stores
information about header and body of the trigger.
SELECT * FROM
user_triggers WHERE trigger_name = 'Before_Update_Stat_product';
The above sql query
provides the header and body of the trigger 'Before_Update_Stat_product'.
You can drop a trigger
using the following command.
DROP TRIGGER
trigger_name;
CYCLIC CASCADING in a
TRIGGER
This is an undesirable
situation where more than one trigger enter into an infinite loop. while
creating a trigger we should ensure the such a situtation does not exist.
The below example shows
how Trigger's can enter into cyclic cascading.
Let's consider we have two tables 'abc' and 'xyz'. Two triggers are created.
1) The INSERT Trigger, triggerA on table 'abc' issues an UPDATE on table 'xyz'.
2) The UPDATE Trigger, triggerB on table 'xyz' issues an INSERT on table 'abc'.
Let's consider we have two tables 'abc' and 'xyz'. Two triggers are created.
1) The INSERT Trigger, triggerA on table 'abc' issues an UPDATE on table 'xyz'.
2) The UPDATE Trigger, triggerB on table 'xyz' issues an INSERT on table 'abc'.
In such a situation, when
there is a row inserted in table 'abc', triggerA fires and will update table
'xyz'.
When the table 'xyz' is updated, triggerB fires and will insert a row in table 'abc'.
This cyclic situation continues and will enter into a infinite loop, which will crash the database.
When the table 'xyz' is updated, triggerB fires and will insert a row in table 'abc'.
This cyclic situation continues and will enter into a infinite loop, which will crash the database.
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