Sunday, 16 September 2012

SQL Server Basics of Cursors | SQL Server Different Types of Cursors



SQL Server Basics of Cursors

Cursor is a database objects to retrieve data from a result set one row at a time, instead of the T-SQL commands that operate on all the rows in the result set at one time. We use cursor when we need to update records in a database table in singleton fashion means row by row.

Life Cycle of Cursor

1.     Declare Cursor

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

2.     Open

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

3.     Fetch

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

4.     Close

After data manipulation, we should close the cursor explicitly.

5.     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,'Ramesh',12000,'Lucknow')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Ramdeo',25000,'Bihar')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Sanjay',22000,'surat')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Sonu',22000,'Pune')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,’Pune’)
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 

Summary
In this article I try to explain the basic of Cursor in SQL Server with a simple example. I hope after reading this article you will be able to understand cursors in Sql Server. I would like to have feedback from my blog readers. Please post your feedback, question, or comments about this article.


SQL Server Different Types of Cursors

A Cursor 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 in the above.

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

1.     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 shou|ld use it only if scrolling is required and other types of cursors are not suitable.

You can't update, delete data using static cursor. It is not sensitive to any changes to the original data source. By default static cursors are scrollable.

2.     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.

3.     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.

4.     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,'Ramesh Kumar',12000,'Lucknow')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Ramdeo',25000,'Bihar')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Sanjay',22000,'surat')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Hemant',22000,'Pune')
INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,’Pune’)
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='Ramesh Kumar'
 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='Ramdeo'
 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='Sanjay'
 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='Ramesh kumar'
 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='Hemant'
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='Ramdeo'
 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 


cursor optimization tips. 

Close cursor when it is not required.
You shouldn’t forget to deallocate cursor after closing it.
You should fetch least number of records.
You should use FORWARD ONLY option when there is no need to update rows. 

disadvantages/limitation of the cursor. 

Cursor requires a network roundtrip each time it fetches a record, thus consume network resources.
While data processing, it issues locks on part of the table, or on the whole table.
  • Uses more resources because Each time you fetch a row from the cursor, it results in a network roundtrip
  • There are restrictions on the SELECT statements that can be used.
  • Because of the round trips, performance and speed is slow


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Summary
In this article I try to explain the types of Cursor in SQL Server with a simple example. I hope after reading this article you will be able to understand different types of cursors in Sql Server. I would like to have feedback from my blog readers. Please post your feedback, question, or comments about this article.



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