sql查詢 —— 分組

-- 分組
      -- group by
      -- 分組只有與聚合函數一起使用才能發揮作用
      -- 分組只限於字段分明 例如 性別 ，部門，
      --列出所有性別
      select gender from student group by gender;
      --列出所有性別 及數量
      select gender,count(*) from student group by gender;

      --group_concat
      -- 統計的數量
      --列出所有性別人名
      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;
      --列出所有性別分別的數量和分別的姓名
      select gender as '性別',count(*) as '數量',group_concat(name) from student group by gender;
      --列出所有年齡大於10，性別，數量及分別的姓名
      select gender as '性別',count(*) as '數量',group_concat(name) from student where age>10 group by gender;


      -- having
      -- having 是在查詢結果中，再查詢
      -- 查詢平均年齡超過20的性別 及姓名
      select gender,group_concat(name),avg(age) from student group by gender having avg(age)>20;
      -- 查詢每種性別人數大於2的，
      select gender,group_concat(name) from student group by gender having count(*)>2;