子查询

就是在简单查询中嵌套查询(where子句里面嵌套)

select name from tablename where id in (
	select id from tablename where num in (100,101)
)

可以一直嵌套下去

但是由于性能的限制,实际上不能嵌套太多的子查询

当然,不一定是in,其他也行

或者在select子句里进行嵌套

select name,(
    	select * count(distinct name) from tablename
    ) as other
from tablename
order by name;

在from子句里面嵌套

select * from (select id,name from tablename order by id) as other

还可以在exists等子句内嵌套