1.微博表blogs：
微博评论表blogs_comm：
两表联合查询将微薄信息跟评论数一起列出的sql语句：
select b.*,count(c.bid) as num from blogs as b left join blogs_comm as c on b.id=c.bid group by c.bid order by b.id
结果：
2.查询一个表t1中某字段(name)同时出现两次的字段名：
表t1数据如下：
select count(1) as num,name from t1 group by name having num=2;
注：如果要对两个字段进行排重，就应该这样写：
select count(1) from t1 group by concat(name,time);
结果：
查询一个表图t1中某字段(name)同时出现两次的记录：
select * from t1 as t where 2=(select count(1) from t1 where t1.name=t.name);
结果：
3.distinct去除重复值：
如下表t1：去除重复的uid(重复的只保留前一条)
(1)select *,count(distinct(uid)) from t1 group by uid;
结果：
(2).如果想在重复的列表中按一定的顺序，只取其中一条：
select *,count(distinct(uid)) from(select * from t1 order by time desc) as t2 group by uid;
结果：
4.一条语句删除多个表的数据：
delete b,c from blogs as b left join comment as c on b.id=c.wid left join collect as l on b.id=l.wid where b.id=27;
5.一条sql语句查询a表主键id关联b表sid的多条记录，并按b表的主键id倒叙排列，同时只显示a表中有的记录：
select *,a.id as aid from a left join (select * from b order by id desc) as i on a.id=i.sid group by i.sid;