sql查询 —— 分组

 -- 分组
       -- group by
       -- 分组只有与聚合函数一起使用才能发挥作用
       -- 分组只限于字段分明 例如 性别 ，部门，
       --列出所有性别
       select gender from student group by gender;
       --列出所有性别 及数量
       select gender,count(*) from student group by gender;

       --group_concat
       -- 统计的数量
       --列出所有性别人名
       select gender,group_concat(name) from student  group by gender;
       --列出所有男性人名
       select gender,group_concat(name) from student where gender='man' group by gender;
       --列出所有性别分别的数量和分别的姓名
       select gender as '性别',count(*) as '数量',group_concat(name) from student group by gender;
       --列出所有年龄大于10，性别，数量及分别的姓名
       select gender as '性别',count(*) as '数量',group_concat(name) from student where age>10 group by gender;

       -- having
       -- having 是在查询结果中，再查询
       -- 查询平均年龄超过20的性别 及姓名
       select gender,group_concat(name),avg(age) from student group by gender having avg(age)>20;
       -- 查询每种性别人数大于2的，
       select gender,group_concat(name) from student group by gender having count(*)>2;