Question

我要向我的SQL提出复杂的问题。如果工会的第一部分无效,我想宣布无效;否则,我要回去工会的结果。例:

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

我的法典行不通。我有以下错误:

Operand should contain 14 column(s)

是否有任何想法来纠正这种情况?

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 ) )

感谢。

Answer 1

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

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