A table name as a variable

Question

I am trying to execute this query:

declare @tablename varchar(50) set @tablename = 'test' select * from @tablename 

This produces the following error:

Msg 1087, Level 16, State 1, Line 5

Must declare the table variable "@tablename".

What's the right way to have the table name populated dynamically?

Answer 1

For static queries, like the one in your question, table names and column names need to be static.

For dynamic queries, you should generate the full SQL dynamically, and use sp_executesql to execute it.

Here is an example of a script used to compare data between the same tables of different databases:

Static query:

SELECT * FROM [DB_ONE].[dbo].[ACTY] EXCEPT SELECT * FROM [DB_TWO].[dbo].[ACTY] 

Since I want to easily change the name of table and schema, I have created this dynamic query:

declare @schema varchar(50) declare @table varchar(50) declare @query nvarchar(500)  set @schema = 'dbo' set @table = 'ACTY'  set @query = 'SELECT * FROM [DB_ONE].[' + @schema + '].[' + @table + '] EXCEPT SELECT * FROM [DB_TWO].[' + @schema + '].[' + @table + ']'  EXEC sp_executesql @query 

Since dynamic queries have many details that need to be considered and they are hard to maintain, I recommend that you read: The curse and blessings of dynamic SQL

Answer 2

Change your last statement to this:

EXEC('SELECT * FROM ' + @tablename) 

This is how I do mine in a stored procedure. The first block will declare the variable, and set the table name based on the current year and month name, in this case TEST_2012OCTOBER. I then check if it exists in the database already, and remove if it does. Then the next block will use a SELECT INTO statement to create the table and populate it with records from another table with parameters.

--DECLARE TABLE NAME VARIABLE DYNAMICALLY DECLARE @table_name varchar(max) SET @table_name =     (SELECT 'TEST_'             + DATENAME(YEAR,GETDATE())             + UPPER(DATENAME(MONTH,GETDATE())) )  --DROP THE TABLE IF IT ALREADY EXISTS IF EXISTS(SELECT name           FROM sysobjects           WHERE name = @table_name AND xtype = 'U')  BEGIN     EXEC('drop table ' +  @table_name) END  --CREATES TABLE FROM DYNAMIC VARIABLE AND INSERTS ROWS FROM ANOTHER TABLE EXEC('SELECT * INTO ' + @table_name + ' FROM dbo.MASTER WHERE STATUS_CD = ''A''')