举个例子说明：
select d.id, d.uid,d.username,d.dateline, d.message,d.replynum, c.picid, c.filename from doing d left join(
select a.*, b.filename from picture a left join my_picture b on (a.id = b.id and a.mid = b.mid)
) c
on c.doid = d.doid limit 1, 20;
解析： 左连接是左边的表数据全部保留，右边的表选择符合条件的数据，拼接成一个数据集（表）。
select a.*, b.filename from picture a left join my_picture b on (a.id = b.id and a.mid = b.mid)这句选取出来的当做一个c表。
c表和d表作左连接，相当于：
select d.id, d.uid,d.username,d.dateline, d.message,d.replynum, c.picid, c.filename from doing d left join c on c.doid = d.doid limit 1, 20;
多个表左连接一样，不断的接下去就好了，如下：
select * from
t4 left join t1 on t4.a=t1.a
left join t2 on t4.b=t2.b and t1.id=t2.id
left join t3 on t4.c=t3.c and t2.id=t3.id