在「WHERE IN」子句中的sql查詢

Question

select * from tablename 
    where id in(select id from tablename2 where condition UNION select -1) 

可以使用select -1，就好像內部查詢不會導致任何錯誤一樣。這是可行的或不可行？

Answer 1

如果查詢不返回任何內容，它不會給出錯誤。它只是返回一個空的結果集。

Answer 2

恕我直言，inner-select很不理想根據你發佈SQL

（慢），an inner join會做的伎倆

select * 
from tablename as t1 
inner join tablename2 as t2 
on t1.id=t2.id 
where condition; --- your condition 

Answer 3

如果你不得不把它與一個子查詢，那麼正確的方式完成要做到這一點可能是：

SELECT * 
FROM tablename AS t1 
WHERE EXISTS 
    (SELECT id 
    FROM tablename2 AS t2 
    WHERE conditions)