sql查询 —— 分组 

 1 -- 分组
 2       -- group by
 3       -- 分组只有与聚合函数一起使用才能发挥作用
 4       -- 分组只限于字段分明 例如 性别 ，部门，
 5       --列出所有性别
 6       select gender from student group by gender;
 7       --列出所有性别 及数量
 8       select gender,count(*) from student group by gender;
 9 
10       --group_concat
11       -- 统计的数量
12       --列出所有性别人名
13       select gender,group_concat(name) from student  group by gender;
14       --列出所有男性人名
15       select gender,group_concat(name) from student where gender='man' group by gender;
16       --列出所有性别分别的数量和分别的姓名
17       select gender as '性别',count(*) as '数量',group_concat(name) from student group by gender;
18       --列出所有年龄大于10，性别，数量及分别的姓名
19       select gender as '性别',count(*) as '数量',group_concat(name) from student where age>10 group by gender;
20 
21 
22       -- having
23       -- having 是在查询结果中，再查询
24       -- 查询平均年龄超过20的性别 及姓名
25       select gender,group_concat(name),avg(age) from student group by gender having avg(age)>20;
26       -- 查询每种性别人数大于2的，
27       select gender,group_concat(name) from student group by gender having count(*)>2;