Explore the SQL Server Cursors basics

Explore the SQL Server Cursors basics

Cursor is the database object in MS SQL Server to retrieve the data from the result set one row at a time either in backward direction or forward direction. Generally in T-SQL command it will direct operates on all the rows of result at one time, so when ever we want to process based on the condition 1 by 1 for our process or data is dependent on the previous actions cursor is the suitable feature to use.

Now lets see the Life cycle of the cursor as below

Declare the cursor

Like any other object we need to declare the cursor first, we can declare the cursor by defininf SQL statement which returs a result set.

Open the cursor

Once cursor is declared we can open and populate by executing the SQL statement defined in the cursors.

Fetch records one by one from the Cursor

One cursor is opened we can fetch rows one by one or in a block for the data manipulation.

Close Cursor

One we have fetched all the rows and manipulated data as required we need to close the cursor explicitly.

Deallocate Cursor

One we have close the cursor at the end, we need to delete the cursor definition and free all the system resources associated with the cursor.

Cursor Syntax

Declare Cursor SQL Command is used to declare the cursor with provided options. There are some options we need to use while declaring the curosr which impact on the performance of cursor.

The complete syntax of declaring a cursor is as below


DECLARE name_of_cursor CURSOR
[LOCAL | GLOBAL] -- Local or global define the scope of the cursor, default it is local only
[FORWARD_ONLY | SCROLL] -- It defines the direction of the cursor either forward or backward
[STATIC | KEYSET | DYNAMIC | FAST_FORWARD] -- It define the basic type of cursor
[READ_ONLY | SCROLL_LOCKS | OPTIMISTIC] -- It define the locks
FOR Select_Query -- Here you can define select or update statement as required
FOR UPDATE [field1,field2,field3 ....., fieldn] -- Here provide the list of fields/columns to updated

Syntax to Open a Cursor

A SQL cursor can be either opened locally or globally. By default it is opened locally.

OPEN [GLOBAL] name_of_cursor

Syntax to Fetch rows from Cursor

Fetch statement provides different different options to retrieve the rows from the cursor. Default NEXT option is applied.


FETCH [NEXT|PRIOR|FIRST|LAST|ABSOLUTE n|RELATIVE n]
FROM [GLOBAL] name_of_cursor
INTO @Name_Of_Variable[1,2,3,4, .....n]

Syntax to Close the Cursor

Close statement closed the cursor explicitly.


CLOSE name_of_cursor -- Once it is closed then it can be reopen

Syntax to Deallocate Cursor at the end

Deallocate statement delete the cursor definition and free all the system resources associated with the cursor.

DEALLOCATE name_of_cursor -- Once cursor is deallocated it can not be open again.

See the sample data as below from which we will retrieve the data using cursor.

The full cursor example

SET NOCOUNT ON

DECLARE @ID int
DECLARE @Title varchar(100)
DECLARE @ISBN varchar(100)
Declare @Price Decimal

DECLARE all_book CURSOR
STATIC FOR
SELECT Id,Title,Isbn,Price from Books

OPEN all_book
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM all_book INTO @ID,@Title,@ISBN,@Price

WHILE @@Fetch_status = 0
BEGIN
Print 'Id :'+Convert(VarChar(20),@ID) + ', Price :'+Convert(VarChar(20),@Price)+ ', Title :'+@Title + ', ISBN :'+@ISBN

FETCH NEXT FROM all_book INTO @ID,@Title,@ISBN,@Price
END
END

CLOSE all_book
DEALLOCATE all_book
SET NOCOUNT OFF

When you run the cursor you will see the below out put.

Summary

in this article we have see the basic concept and syntax of the SQL Server Cursor. I hope it will help you to understand the cursors. i would like to have feedback from my blog readers