SQL Server Cursor Example

Problem

In my T-SQL code I always use set based operations.  I have been told these types of operations are what SQL Server is designed to process and it should be quicker than serial processing.  I know cursors exist but I am not sure how to use them.  Can you provide some cursor examples?  Can you give any guidance on when to use cursors?  I assume Microsoft created them for a reason so they must have a place where they can be used in an efficient manner.

Solution

In some circles cursors are never used, in others they are a last resort and in other groups they are used regularly.  In each of these camps they have different reasons for their stand on cursor usage.  Regardless of your stand on cursors they probably have a place in particular circumstances and not in others.  So it boils down to your understanding of the coding technique then your understanding of the problem at hand to make a decision on whether or not cursor based processing is appropriate or not.  To get started let's do the following:

• Look at an example cursor
• Break down the components of the cursor
• Provide additional cursor examples
• Analyze the pros and cons of cursor usage

Example SQL Server Cursor

Here is an example cursor from tip Simple script to backup all SQL Server databases where backups are issued in a serial manner:

DECLARE @name VARCHAR(50) -- database name  DECLARE @path VARCHAR(256) -- path for backup files  DECLARE @fileName VARCHAR(256) -- filename for backup  DECLARE @fileDate VARCHAR(20) -- used for file name
SET @path = 'C:\Backup\' 
SELECT @fileDate = CONVERT(VARCHAR(20),GETDATE(),112)
DECLARE db_cursor CURSOR FOR 
SELECT name
FROM MASTER.dbo.sysdatabases
WHERE name NOT IN ('master','model','msdb','tempdb') 
OPEN db_cursor  
FETCH NEXT FROM db_cursor INTO @name  
WHILE @@FETCH_STATUS = 0  
BEGIN  
       SET @fileName = @path + @name + '_' + @fileDate + '.BAK' 
       BACKUP DATABASE @name TO DISK = @fileName 

       FETCH NEXT FROM db_cursor INTO @name   END  

CLOSE db_cursor  
DEALLOCATE db_cursor

SQL Server Cursor Components

Based on the example above, cursors include these components:

• DECLARE statements - Declare variables used in the code block
• SET\SELECT statements - Initialize the variables to a specific value
• DECLARE CURSOR statement - Populate the cursor with values that will be evaluated
  • NOTE - There are an equal number of variables in the DECLARE <cursor_name> CURSOR FOR statement as there are in the SELECT statement.  This could be 1 or many variables and associated columns.
• OPEN statement - Open the cursor to begin data processing
• FETCH NEXT statements - Assign the specific values from the cursor to the variables
  • NOTE - This logic is used for the initial population before the WHILE statement and then again during each loop in the process as a portion of the WHILE statement
• WHILE statement - Condition to begin and continue data processing
• BEGIN...END statement - Start and end of the code block
  • NOTE - Based on the data processing multiple BEGIN...END statements can be used
• Data processing - In this example, this logic is to backup a database to a specific path and file name, but this could be just about any DML or administrative logic
• CLOSE statement - Releases the current data and associated locks, but permits the cursor to be re-opened
• DEALLOCATE statement - Destroys the cursor