Question

Consider a simple join:

Select TableB.* 
From TableA 
Inner Join TableB 
    On TableB.ID = TableA.ID

What I want to do is decide which table to join to depending on a parameter. Although the following syntax is not valid, I wrote it just to illustrate what I am after:

Select TableD.* 
From TableA 
Inner Join 
    [If @useTableC = 1 Then Join to TableC Else Join to TableB] As TableD

Both TableB and TableC have identical columns.

How can I create this kind of join. Please be aware that this example is actually a small portion of a much larger query so you cannot just use If…Else statements.

Thanks a lot!

Answer 1

Usually I’d prefer to rewrite in more than one statement, but if you really need it:

SELECT td.*
FROM TableA ta
JOIN (
    SELECT tc.* FROM TableC Where @useTableC = 1
    UNION ALL
    SELECT tb.* FROM TableB Where @useTableC = 0
) td ON ( /* JOIN CONDITION MISSING */)