Multiple ONs and JOINs

Question

我了解到，我可以重写一个查询，比如

SELECT d
FROM tbl1
WHERE a in (
    SELECT  b
    FROM tbl2
    WHERE c = "Something"
)

进入（在我看来更快）查询：

SELECT d
FROM tbl1 LEFT JOIN tbl2
ON tbl1.a = tbl2.b
WHERE tbl2.c = "Something"

但是我在重写以下查询时遇到问题：

SELECT name, nick
FROM cu
WHERE uid IN (
    SELECT friend
    FROM cf
    WHERE user IN (
        SELECT uid
        FROM cu
        WHERE sid = "abc"
    )
)

编辑：

刚刚找到它：左联接优化

Answer 1

这应该是等效的：

select cu.*
from cu join
     cf
     on cu.uid = cf.friend join
     cu cu2
     on cu2.uid = cf.user
where cu2.sid = 'abc';

请注意，您只需要内部联接即可实现此目的。