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;