Conditional JOIN Statement SQL Server

Question

Is it possible to do the following:

IF [a] = 1234 THEN JOIN ON TableA 
ELSE JOIN ON TableB

If so, what is the correct syntax?

Answer 1

I disagree with the solution suggesting 2 left joins. I think a table-valued function is more appropriate so you don't have all the coalescing and additional joins for each condition you would have.

CREATE FUNCTION f_GetData (
    @Logic VARCHAR(50)
) RETURNS @Results TABLE (
    Content VARCHAR(100)
) AS
BEGIN
    IF @Logic = '1234'
        INSERT @Results
            SELECT Content
            FROM Table_1
    ELSE
        INSERT @Results
            SELECT Content
            FROM Table_2
    RETURN
END
GO

SELECT *
FROM InputTable
    CROSS APPLY f_GetData(InputTable.Logic) T