Friday, 15 July 2011

Using Temp tables in SQLServer and Adding columns dynamically

While creating an stored procedure it’s often to create temporary tables to hold temporary operational data. To achieve this there are options available in SQL Server you can use Cursors, #Temp tables or Table variables. In this post we’ll see how to use the Temporary tables, what are the necessary step to be taken while using them and moreover sometimes it’s required to add column in Temporary tables dynamically. I hope this article will help you achieving all the basics operations with Temporary tables.
Declaring Temporary Table:
CREATE TABLE #Mytemp(Col1 nvarchar(100), Col2 int)
Now before using this statement in your SQL always place a check if table already exists in TempDB. (Temporary tables are getting created in TempDB in SQLServer.) Drop that table first otherwise you’ll see error when you run the SQL. Well its suggested you should Drop all the temporary objects you’ve created in your SQL for cleanup.
IF EXISTS
 (
 SELECT *
 FROM tempdb.dbo.sysobjects
 WHERE ID = OBJECT_ID(N'tempdb..#Mytemp')
 )
 BEGIN
 DROP TABLE #Mytemp
 END
Adding records into #Temp table:
INSERT INTO #Mytemp Select *
 from [SomeTable]
Note: Always create the temp table structure based on the query that inserts the data in #Temp table.
Looping through the Records:
Experts don’t recommend using Cursors due to slow performance. So #Temp tables can be replacements to Cursors. Let see how to iterate through rows in temp table.
DECLARE @Var1 nvarchar(100)
WHILE (SELECT COUNT(*) from #Mytemp) >0
 BEGIN
 SELECT TOP 1 @Var1 = Col1 from #Mytemp
 --Do something here


 EXEC(@DynamicSQL)
 --
  Delete #Mytemp Where  @Var1 = Col1
 END
These are the basics things that required to deal with temp table variables.
Now let’s do some interesting operation on #Temp tables.
Adding Columns in #Temp table dynamically:
Temp tables allow changing in their structure when required once they get created.
DECLARE @ColName nvarchar(100)
DECLARE @DynamicSQL nvarchar(250)
SET @ColName='newColumn'
SET @DynamicSQL = 'ALTER TABLE #Mytemp ADD ['+ CAST(@ColName AS NVARCHAR(100)) +'] NVARCHAR(100) NULL'

EXEC(@DynamicSQL)

Adding Identity Column into #Temp Table:
CREATE TABLE #tmp(ID INT IDENTITY(1,1), Col1 nvarchar(100), Col2 int)
Or you can later add it by using the ALTER statement
ALTER TABLE #Temp ADD AutoID INT IDENTITY(1,1);
SQL is fun!!

2 comments:

  1. Thanks a lot, this helped me lot in solving my issue.

    ReplyDelete
  2. good post. simple (and short) yet clear :)

    ReplyDelete