having 与where 的异同点

having 与where 的异同点:
where针对表中的列发挥作用，查询数据
having对查询结果中的列发挥作用，筛选数据
 
#查询本店商品价格比市场价低多少钱，输出低200元以上的商品
select    goods_id,   market_price - shop_price    as s     from goods    having   s>200 ;    //这里不能用where因为s是查询结果，而where只能对表中的字段名筛选
 
如果用where的话则是：
select goods_id,goods_name from goods where market_price - shop_price > 200;
 

#同时使用where与having
select    cat_id,  market_price - shop_price   as s    from goods    where cat_id = 3    having s > 200;
 
#查询积压货款超过2万元的栏目，以及该栏目积压的货款
select      cat_id,   sum(shop_price * goods_number) as t     from goods    group by    cat_id    having s > 20000
 
#查询两门及两门以上科目不及格的学生的平均分
      思路：
        #先计算所有学生的平均分
         select    name,    avg(score) as pj    from stu     group by name;
        #查出所有学生的挂科情况
        select name,score<60 from stu;
#这里score<60是判断语句，所以结果为真或假，mysql中真为1假为0
        #查出两门及两门以上不及格的学生
        select  name,   sum(score<60) as    gk    from stu    group by name     having gk > 1;
 
        #综合结果
        select name,sum(score<60) as gk,  avg(score) as pj    from stu    group by name   having gk >1;