sql嵌套语句问题，急急急

select s.sn
from s inner join sc on s.s#=sc.s#
and exists (
select 1 from c where c.c#=sc.c#);

在MySQL中，CROSS JOIN语法上等价于INNER JOIN (它们可以彼此代替。在标准SQL中，它们不等价。INNER JOIN结合ON子句使用；CROSS JOIN 用于其它地方)

参考自：
MySQL如何优化嵌套Join(一) http://www.data.5helpyou.com/article272.html
MySQL如何优化嵌套Join(二) http://www.data.5helpyou.com/article274.html

SELECT S.SN
FROM S，SC
WHERE S.S# = SC.S#
AND SC.C# in (
SELECT C.C# FROM C WHERE C.C# = SC.C#
)

http://www.cnblogs.com/losesea/archive/2012/10/10/2718093.html