Database : SQL Server Basics of Cursors

The main advantage of stored procedure is, to execute T-SQL statements in less time than the similar set of T-SQL statements is executed individually. The reason to take less time is that the query execution plan for the stored procedures is already stored in the "sys.procedures" system defined view.

The recompilation process of stored procedure is like as compilation process and also reduce Sql Server performance. Stored procedure with recompilation option was introduced in Sql Server 2005. We should recompile stored procedure in following cases

Changing to the schema (means adding/dropping columns, constraints, rules, index, trigger etc) of the tables or referenced table(s) or view(s).
Updating the statistics used by the execution plan of stored procedure

We have two options for stored procedure recompilation

Recompile option at the time of Creation

In this we create a stored procedure with RECOMPILE option. When we call this procedure than every time this procedure will be recompile before executing.


 CREATE PROCEDURE usp_InsertEmployee
WITH RECOMPILE
@flag bit output,-- return 0 for fail,1 for success
@EmpID int,
@Name varchar(50),
@Salary int,
@Address varchar(100)
AS
BEGIN
 BEGIN TRANSACTION 
 BEGIN TRY
 Insert into Employee(EmpID,Name,Salary,Address) Values(@EmpID,@Name,@Salary,@Address);
 set @flag=1;
 commit TRANSACTION;
 END TRY 
BEGIN CATCH
 rollback TRANSACTION;
 set @flag=0;
 END CATCH
END


 Declare @flag bit
--Now Execute this procedure. Every time this procedure will be recompiled
EXEC usp_InsertEmployee @flag output,1,'Deepak',14000,'Noida'
if @flag=1
 print 'Successfully inserted'
else
 print 'There is some error'

Recompile option at the time of Execution

In this we call a stored procedure with RECOMPILE option. Hence this stored procedure will be compiled only when we use RECOMPILE option at the time of calling. This is the best option for stored procedure recompilation.


 CREATE PROCEDURE usp_InsertEmployee
@flag bit output,-- return 0 for fail,1 for success
@EmpID int,
@Name varchar(50),
@Salary int,
@Address varchar(100)
AS
BEGIN
 BEGIN TRANSACTION 
 BEGIN TRY
 Insert into Employee(EmpID,Name,Salary,Address) Values(@EmpID,@Name,@Salary,@Address);
 set @flag=1;
 commit TRANSACTION;
 END TRY 
BEGIN CATCH
 rollback TRANSACTION;
 set @flag=0;
 END CATCH
END


 Declare @flag bit
--Now Execute this procedure with RECOMPILE option, if you want to recompile its execution plan
EXEC usp_InsertEmployee @flag output,2,'Jitendra',15000,'Noida' WITH RECOMPILE
if @flag=1
 print 'Successfully inserted'
else
 print 'There is some error'

Note

Creating the stored procedure by using "WITH RECOMPILE" option force the SQL Server to recompile the stored procedure every time when it is called.
Call the stored procedure by using "WITH RECOMPILE" option in the EXEC command.
Altering the procedure will cause the SQL Server to create a new execution plan
If SQL Server is restarted or stopped then all the execution plans will be flush from server cache and recreated when the stored procedure is executed after restarting the server.
The "Sp_recompile" system defined stored procedure can be called to refresh the query execution plan for a particular stored procedure

ursor allow us to retrieve data from a result set in singleton fashion means row by row. Cursor are required when we need to update records in a database table one row at a time. I have already explained the basic of cursor.

A Cursor impacts the performance of the SQL Server since it uses the SQL Server instances' memory, reduce concurrency, decrease network bandwidth and lock resources. Hence it is mandatory to understand the cursor types and its functions so that you can use suitable cursor according to your needs.

You should avoid the use of cursor. Basically you should use cursor alternatives like as WHILE loop, sub queries, Temporary tables and Table variables. We should use cursor in that case when there is no option except cursor.

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

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