从两个表中查询点赞数最高的5条博客信息，要求显示字段：（博文id，标题，点赞数，会员名）
Select b.id,b.title,b.pcount,u.name from blog b,users,u where u.id=b.uid order by pcount desc limit 5
统计每个会员的发表博文数量（降序），要求显示字段（会员id号，姓名，博文数量）
Select u.id,u.name count(b.id) n from user u join left blog b on b.uid=u.id group by u.id order by n desc
获取会员的博文平均点赞数量最高的三位。显示字段（会员id，姓名，平均点赞数）
Select u.id,u.name avg(b.pcount) n from user u join left blog b on b.uid=u.id group by u.id order by n desc limit 3
删除没有发表博文的所有会员信息
Delete * from user where id not in (select distinct uid from blog)