PL/SQL Triggers

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

Syntax for Creating a Trigger

 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. 

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.

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

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

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'.

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.

PL/SQL Procedures, Functions and Packages

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

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.

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.

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.

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.

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.

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.

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. 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:

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’.

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> /

PL/SQL 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: Application

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.

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.

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.

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.

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.

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;

Explicit Cursor, Lets Look at the example below

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.

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.

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.