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GROUP BY子句，HAVING子句。
聚合函数：COUNT();SUM();AVG();MAX();MIN().
1.GROUP BY子句的应用
select state from MemberDetails group by state; --严格来说，select distinct也能完成这个任务。NULL值也能够被列出。 select state from MemberDetails where state in(‘Mega State‘,‘Goldern State‘,‘New State‘) group by state; --也可以包含一个where子句，必须放在group by之前
select street,city,state from MemberDetails group by street,City,State; --多个列的情况。列举出这3列的唯一组合
select city,state from MemberDetails group by state; --查询错误。因为city列没有包含在聚合函数或GROUP BY子句中
2.汇总和聚合数据
（1）COUNT()函数
select count(*) from MemberDetails; select count(street) from MemberDetails; select count(city),count(state) from MemberDetails;
select count(distinct CategoryId) from FavCategory; --聚合函数中的distinct选项 select city,count(LastName) from MemberDetails; --错误。city可能返回多行，但是count(city)只返回一行
select state,count(lastname) from MemberDetails group by state; --列举每类state中lastname列的数目
（2）SUM()函数
select sum(DVDPrice*1.1) from Films where AvailableOnDVD=‘Y‘ and DVDPrice<10;
（3）AVG()函数
select Category,AVG(DVDPrice) FROM Films inner join Category on films.CategoryId=Category.Category where AvailableOnDVD=‘Y‘ group by Category;
（4）MAX(),MIN()函数
select max(lastname),min(lastname) from MemberDetails where MemberDetails>3;
3.HAVING子句和GROUP BY子句的综合应用
HAVING子句是针对每个分组的，过滤掉了不匹配条件的分组。而WHERE子句是针对记录的，过滤掉了不匹配条件的记录。
select Category,count(FavCategory.CategoryId) as popularity from FavCategory inner join Category on FavCategory.CategoryId=Category.CategoryId group by Category.Category having count(FavCategory.CategoryId)>3 order by popularity desc; --仅列举那些被3个以上成员喜爱的电影的种类，并且按降序排列
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原文地址：http://www.cnblogs.com/wy1290939507/p/4532037.html
