group by--------按**分组查询
SELECT rank, MIN(weight)as min_weight FROM userinfo GROUP BY rank
对分组的值进行过滤需要使用HAVING,where只能对特定行的值进行过滤(另一种理解:分组前过滤:where;分组后过滤:having)
SELECT rank,MIN(weight)as min_weight FROM userinfo GROUP BY rank HAVING MIN(weight)>60
SELECT rank,COUNT(*)AS num FROM userinfo WHERE weight>50 GROUP BY rank HAVING COUNT(*)>3
顺序
select 语句中的使用顺序