Conditional JOIN Statement SQL Server

I think what you are asking for will work by joining the Initial table to both Option_A and Option_B using LEFT JOIN, which will produce something like this:

Initial LEFT JOIN Option_A LEFT JOIN NULLORInitial LEFT JOIN NULL LEFT JOIN Option_B

Example code:

SELECT i.*, COALESCE(a.id, b.id) as Option_Id, COALESCE(a.name, b.name) as Option_NameFROM Initial_Table iLEFT JOIN Option_A_Table a ON a.initial_id = i.id AND i.special_value = 1234LEFT JOIN Option_B_Table b ON b.initial_id = i.id AND i.special_value <> 1234

Once you have done this, you 'ignore' the set of NULLS. The additional trick here is in the SELECT line, where you need to decide what to do with the NULL fields. If the Option_A and Option_B tables are similar, then you can use the COALESCE function to return the first NON NULL value (as per the example).

The other option is that you will simply have to list the Option_A fields and the Option_B fields, and let whatever is using the ResultSet to handle determining which fields to use.

This is just to add the point that query can be constructed dynamically based on conditions.An example is given below.

DECLARE @a INT = 1235DECLARE @sql VARCHAR(MAX) = 'SELECT * FROM [sourceTable] S JOIN ' + IIF(@a = 1234,'[TableA] A ON A.col = S.col','[TableB] B ON B.col = S.col') EXEC(@sql)--Query will be/*SELECT * FROM [sourceTable] S JOIN [TableB] B ON B.col = S.col*/

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)) ASBEGIN    IF @Logic = '1234'        INSERT @Results            SELECT Content            FROM Table_1    ELSE        INSERT @Results            SELECT Content            FROM Table_2    RETURNENDGOSELECT *FROM InputTable    CROSS APPLY f_GetData(InputTable.Logic) T