1 SELECT
    ...
   2 FROM
   ...
   3 WHERE
   ...
   4 GROUP BY
   ...
   5 HAVING
   ...
   6 ORDER BY
   ...

                  数据源          位置            关键字
  
  WHERE           原始表      ORDER BY语句之前     WHERE
  
  HAVING      分组后的结果集   ORDER BY语句之后    HAVING

#1.查询每个班学生的最大年龄
 SELECT 
       MAX(age),class
  FROM
       STU_CLASS
  GROUP BY CLASS;
# 2.查询每个班中男女生的最大年龄
 SELECT
         MAX(age)，class,sex
  FROM
        STU_CLASS
   GROUP BY
        class,sex;
#3.查询每个班中男女生的平局年龄，且平均年龄大于15岁
SELECT
       AVG（age）,class,sex
FROM 
       STU_CLASS
GROUP BY
       class,sex
 HAVING
        avg(age) > 15;