如果存在，则子查询返回超过1行

Question

将WHERE id IN与条件子查询一起使用时，为什么子查询必须返回1行？为什么此查询适用于limit 1，但不是没有？我怎样才能做到这一点？

SQLFiddle：http://sqlfiddle.com/#!2/638d30/1

/* remove comment around limit 1 and this query works */

SELECT * FROM ForgeRock WHERE `id` IN (
        if(exists (SELECT `fid` FROM `AvailableRock` WHERE `fid` = 2),
           (SELECT `fid` FROM `AvailableRock` WHERE `fid` = 2),
           (SELECT `id` FROM ForgeRock /* LIMIT 1 */)
        )
);

Answer 1

这有效：

select * from ForgeRock where 
(exists(SELECT `fid` FROM `AvailableRock` where `fid` = 1) AND
`id` IN (SELECT `fid` FROM `AvailableRock` where `fid` = 1)) 
OR NOT exists(SELECT `fid` FROM `AvailableRock` where `fid` = 1)

SQLFiddle