Cursors exist because there are situations that row-by-row processing is inevitable. However as they are resource intensive, developers always try to refine their codes and bypass cursors using T-SQL tricks.
Not all scenarios are capable for this replacement, but I have seen many of them! One of the common usages of cursors is inside triggers for processing the Deleted and Inserted tables. At the time of writing this article, SQL Server does not support any mechanism to fire a trigger for each row separately when a DML statement affects more than one row.
As an example, an Insert command is inserting 5 records (from a query) into a table. Your trigger needs to retrieve each PK value from Inserted table and send it to a particular Stored Procedure for some processes. Iterating through a cursor made of PKs might seem the first option. However in this case we can use the power of variables that live within a query, then Dynamic T-SQL Execution finalizes the trick! Let's create necessary objects:
CREATE TABLE [dbo].[Employee](
[EmployeeID] [int] IDENTITY(1,1) NOT NULL,
[EmployeeName] [nvarchar](50) COLLATE Latin1_General_CI_AI NOT NULL,
CONSTRAINT [PK_Employee] PRIMARY KEY CLUSTERED
(
[EmployeeID] ASC
)WITH (IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]// Create Procedure...!
CREATE PROCEDURE [dbo].[usp_Process]
-- Add the parameters for the stored procedure here
@EmpName varchar(50)
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
Print 'Sp is processing..' + @EmpName
END
GO
All we need to do in the trigger is to construct a string of T-SQL commands that contain EXEC usp_Process for each EmpName:
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go
Create TRIGGER [tr1] ON [dbo].[Employee] AFTER INSERT AS
DECLARE @sql VARCHAR(8000)
SET @sql=''
SELECT @sql=@sql+'EXEC usp_Process ''' + EmployeeName + '''; ' FROM Inserted
PRINT 'Trigger is preparing Dynamic T-SQL: ' + @sql
EXEC (@sql)// Add Some Entries
Insert Employee(EmployeeName)
Select 'Bahutik' Union
Select 'Manan' Union
Select 'Tarang' Union
Select 'Kuldeep' Union
Select 'Kejal' Union
Select 'Kunal'
Ouput Message:
Trigger is preparing Dynamic T-SQL: EXEC usp_Process 'Tarang'; EXEC usp_Process 'Manan'; EXEC usp_Process 'Kunal'; EXEC usp_Process 'Kuldeep'; EXEC usp_Process 'Kejal'; EXEC usp_Process 'Bahutik';
Sp is processing..Tarang
Sp is processing..Manan
Sp is processing..Kunal
Sp is processing..Kuldeep
Sp is processing..Kejal
Sp is processing..Bahutik
(6 row(s) affected)
This manner can be used even if you need to send more parameters to the SP.
Note: Please be careful with @sql here because your dynamically-built string cannot exceed 8000 characters. Fortunately SQL Server 2005 developers can benefit from MAX length of VARCHAR and NVARCHAR data types!
Conclusion:
Use the power of T-SQL and SELECT statement whenever you can. They are flexible enough to help us to perform some sort of row-by-row processing faster without bothering the hardware. Experts think about a Cursor as the last option!
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