Question

I want to make a compound query into MySQL. If the first part of the union is null, I want to return null; otherwise, I want to return the result of the union. Example:

IF( select * (select * from t1) as result, result UNION (select * from t2), "");

My line of code is not working. I get the following error:

Operand should contain 14 column(s)

Does anyone have any ideas how to do this corrrectly?

Thanks for the first reply below but it looks too expensive:

(SELECT * FROM t1) 
UNION 
( SELECT * FROM t2 
WHERE EXISTS
    ( SELECT * FROM t1 )
)

My reason is t1 is actually a bunch of UNIONed statements itself. Is there a way to use something similar to an alias? The example below failed because the alias “result” is not carried across the UNION keyword.

SELECT * FROM ( large compound select statement) AS result
UNION
( SELECT * FROM t2
WHERE EXISTS
( SELECT 1 FROM result )
)

Thanks.

Answer 1

( SELECT * FROM t1 )
UNION 
( SELECT * FROM t2 
  WHERE EXISTS
        ( SELECT * FROM t1 )
)