Dharmendra (.Net): Cursor in SQL

Life Cycle of Cursor

Declare Cursor

A cursor is declared by defining the SQL statement that returns a result set.
Open

A Cursor is opened and populated by executing the SQL statement defined by the cursor.
Fetch

When cursor is opened, rows can be fetched from the cursor one by one or in a block to do data manipulation.
Close

After data manipulation, we should close the cursor explicitly.
Deallocate

Finally, we need to delete the cursor definition and released all the system resources associated with the cursor.

Syntax to Declare Cursor

Declare Cursor SQL Comaand is used to define the cursor with many options that impact the scalablity and loading behaviour of the cursor. The basic syntax is given below


 DECLARE cursor_name CURSOR
 [LOCAL | GLOBAL] --define cursor scope
 [FORWARD_ONLY | SCROLL] --define cursor movements (forward/backward)
 [STATIC | KEYSET | DYNAMIC | FAST_FORWARD] --basic type of cursor
 [READ_ONLY | SCROLL_LOCKS | OPTIMISTIC] --define locks
 FOR select_statement --define SQL Select statement
 FOR UPDATE [col1,col2,...coln] --define columns that need to be updated

Syntax to Open Cursor

A Cursor can be opened locally or globally. By default it is opened locally. The basic syntax to open cursor is given below:


 OPEN [GLOBAL] cursor_name --by default it is local

Syntax to Fetch Cursor

Fetch statement provides the many options to retrieve the rows from the cursor. NEXT is the default option. The basic syntax to fetch cursor is given below:


 FETCH [NEXT|PRIOR|FIRST|LAST|ABSOLUTE n|RELATIVE n]
FROM [GLOBAL] cursor_name 
INTO @Variable_name[1,2,..n]

Syntax to Close Cursor

Close statement closed the cursor explicitly. The basic syntax to close cursor is given below:


 CLOSE cursor_name --after closing it can be reopen

Syntax to Deallocate Cursor

Deallocate statement delete the cursor definition and free all the system resources associated with the cursor. The basic syntax to close cursor is given below:


 DEALLOCATE cursor_name --after deallocation it can't be reopen

SQL SERVER – Simple Examples of Cursors


 CREATE TABLE Employee
(
 EmpID int PRIMARY KEY,
 EmpName varchar (50) NOT NULL,
 Salary int NOT NULL,
 Address varchar (200) NOT NULL,
)
GO
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(1,'Mohan',12000,'Noida')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Pavan',25000,'Delhi')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Amit',22000,'Dehradun')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Sonu',22000,'Noida')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,'Gurgaon')
GO
SELECT * FROM Employee


 SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
DECLARE @salary int
 DECLARE cur_emp CURSOR
STATIC FOR 
SELECT EmpID,EmpName,Salary from Employee
OPEN cur_emp
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
 WHILE @@Fetch_status = 0
 BEGIN
 PRINT 'ID : '+ convert(varchar(20),@Id)+', Name : '+@name+ ', Salary : '+convert(varchar(20),@salary)
 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
 END
END
CLOSE cur_emp
DEALLOCATE cur_emp
SET NOCOUNT OFF 



Types of Cursors


Static Cursors

A static cursor populates the result set at the time of cursor creation and query result is cached for the lifetime of the cursor. A static cursor can move forward and backward direction. A static cursor is slower and use more memory in comparison to other cursor. Hence you should use it only if scrolling is required and other types of cursors are not suitable.

No UPDATE, INSERT, or DELETE operations are reflected in a static cursor (unless the cursor is closed and reopened). By default static cursors are scrollable. SQL Server static cursors are always read-only.


Dynamic Cursors

A dynamic cursor allows you to see the data updation, deletion and insertion in the data source while the cursor is open. Hence a dynamic cursor is sensitive to any changes to the data source and supports update, delete operations. By default dynamic cursors are scrollable.


Forward Only Cursors

A forward only cursor is the fastest cursor among the all cursors but it doesn't support backward scrolling. You can update, delete data using Forward Only cursor. It is sensitive to any changes to the original data source.

There are three more types of Forward Only Cursors.Forward_Only KEYSET, FORWARD_ONLY STATIC and FAST_FORWARD.



A FORWARD_ONLY STATIC Cursor is populated at the time of creation and cached the data to the cursor lifetime. It is not sensitive to any changes to the data source.

A FAST_FORWARD Cursor is the fastest cursor and it is not sensitive to any changes to the data source.


Keyset Driven Cursors

A keyset driven cursor is controlled by a set of unique identifiers as the keys in the keyset. The keyset depends on all the rows that qualified the SELECT statement at the time of cursor was opened. A keyset driven cursor is sensitive to any changes to the data source and supports update, delete operations. By default keyset driven cursors are scrollable.



SQL SERVER – Examples of Cursors

 CREATE TABLE Employee
(
 EmpID int PRIMARY KEY,
 EmpName varchar (50) NOT NULL,
 Salary int NOT NULL,
 Address varchar (200) NOT NULL,
)
GO
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(1,'Mohan',12000,'Noida')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Pavan',25000,'Delhi')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Amit',22000,'Dehradun')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Sonu',22000,'Noida')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,'Gurgaon')
GO
SELECT * FROM Employee 



Static Cursor - Example

 SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
DECLARE @salary int
 DECLARE cur_emp CURSOR
STATIC FOR 
SELECT EmpID,EmpName,Salary from Employee
OPEN cur_emp
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
 WHILE @@Fetch_status = 0
 BEGIN
 PRINT 'ID : '+ convert(varchar(20),@Id)+', Name : '+@name+ ', Salary : '+convert(varchar(20),@salary)
 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
 END
END
CLOSE cur_emp
DEALLOCATE cur_emp
SET NOCOUNT OFF 



Dynamic Cursor - Example

 --Dynamic Cursor for Update
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Dynamic_cur_empupdate CURSOR
DYNAMIC 
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Dynamic_cur_empupdate
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Dynamic_cur_empupdate INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Mohan'
 Update Employee SET Salary=15000 WHERE CURRENT OF Dynamic_cur_empupdate
 FETCH NEXT FROM Dynamic_cur_empupdate INTO @Id,@name
 END
END
CLOSE Dynamic_cur_empupdate
DEALLOCATE Dynamic_cur_empupdate
SET NOCOUNT OFF
 Go
Select * from Employee 



 -- Dynamic Cursor for DELETE
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Dynamic_cur_empdelete CURSOR
DYNAMIC 
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Dynamic_cur_empdelete
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Dynamic_cur_empdelete INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Deepak'
 DELETE Employee WHERE CURRENT OF Dynamic_cur_empdelete
 FETCH NEXT FROM Dynamic_cur_empdelete INTO @Id,@name
 END
END
CLOSE Dynamic_cur_empdelete
DEALLOCATE Dynamic_cur_empdelete
SET NOCOUNT OFF
Go
Select * from Employee 



Forward Only Cursor - Example

 --Forward Only Cursor for Update
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Forward_cur_empupdate CURSOR
FORWARD_ONLY
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Forward_cur_empupdate
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Forward_cur_empupdate INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Amit'
 Update Employee SET Salary=24000 WHERE CURRENT OF Forward_cur_empupdate
 FETCH NEXT FROM Forward_cur_empupdate INTO @Id,@name
 END
END
CLOSE Forward_cur_empupdate
DEALLOCATE Forward_cur_empupdate
SET NOCOUNT OFF
 Go
Select * from Employee 



 -- Forward Only Cursor for Delete
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Forward_cur_empdelete CURSOR
FORWARD_ONLY
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Forward_cur_empdelete
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Forward_cur_empdelete INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Sonu'
 DELETE Employee WHERE CURRENT OF Forward_cur_empdelete
 FETCH NEXT FROM Forward_cur_empdelete INTO @Id,@name
 END
END
CLOSE Forward_cur_empdelete
DEALLOCATE Forward_cur_empdelete
SET NOCOUNT OFF
 Go
Select * from Employee 



Keyset Driven Cursor - Example

 -- Keyset driven Cursor for Update
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Keyset_cur_empupdate CURSOR
KEYSET
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Keyset_cur_empupdate
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Keyset_cur_empupdate INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Pavan'
 Update Employee SET Salary=27000 WHERE CURRENT OF Keyset_cur_empupdate
 FETCH NEXT FROM Keyset_cur_empupdate INTO @Id,@name
 END
END
CLOSE Keyset_cur_empupdate
DEALLOCATE Keyset_cur_empupdate
SET NOCOUNT OFF
 Go
Select * from Employee 



 -- Keyse Driven Cursor for Delete
SET NOCOUNT ON
DECLARE @Id int
DECLARE @name varchar(50)
 DECLARE Keyset_cur_empdelete CURSOR
KEYSET
FOR 
SELECT EmpID,EmpName from Employee ORDER BY EmpName
OPEN Keyset_cur_empdelete
IF @@CURSOR_ROWS > 0
 BEGIN 
 FETCH NEXT FROM Keyset_cur_empdelete INTO @Id,@name
 WHILE @@Fetch_status = 0
 BEGIN
 IF @name='Amit'
 DELETE Employee WHERE CURRENT OF Keyset_cur_empdelete
 FETCH NEXT FROM Keyset_cur_empdelete INTO @Id,@name
 END
END
CLOSE Keyset_cur_empdelete
DEALLOCATE Keyset_cur_empdelete
SET NOCOUNT OFF
 Go Select * from Employee