“WHERE IN”子句中的SQL查询

Question

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

使用select -1是否可以，就好像内部查询不会产生任何错误一样。是否可行？

Answer 1

imho，inner-select远非理想（慢）

根据您发布的SQL，an inner join将执行此操作

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

Answer 2

如果必须使用子查询完成它，那么正确的方法可能是：

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

Answer 3

如果查询没有返回任何内容，则不会出错。它只返回一个空的结果集。