Can I write a query has a conditional table selection

Question

We have 2 tables with identical structure and based on a variable I want to choose which table to select on with out having to write 2 queries in my procedure.

Is this possible?

I tried

declare @table int
set @table = 1 

Select orderID, Quantity 
from case when @table = 1 then tblOrders else tblSubscriptionOrders end
where filled = 0

But that did not work

Answer 1

You would need to use dynamic SQL for this (assuming you want to scale it to more than just 2 tables), which would work but is suboptimal as SQL will not generate statistics for it and have a harder time optimizing the query.

declare @table sysname
declare @SQL varchar(1000)

set @table = 'MyTable'
SET @SQL='SELECT orderID, Quantity FROM ' + QUOTENAME(@table) + ' WHERE filled=0'

exec sp_executesql @SQL

or, in a stored procedure:

CREATE PROCEDURE p_ConditionalSelect @table sysname
as

declare @SQL varchar(1000)

set @table = 'MyTable'
SET @SQL='SELECT orderID, Quantity FROM ' + QUOTENAME(@table) + ' WHERE filled=0'

exec sp_executesql @SQL

Answer 2

If it's just two tables you could do:

Declare @table = 1

SELECT *
FROM Table1
WHERE <stuff>
AND @Table = 1

UNION ALL

SELECT *
FROM Table2
WHERE <stuff>
AND @Table = 2

The filter on @table will result in only one of the two halves showing data.