mysql之分组查询

mysql分组查询字段有group by 和Having

分组查询用例1

将所有人员按性别分组，分别统计男性和女性的人数

语法

select 字段名 from 表名 group by 分组条件

select gender,count(*) from tb_user group by gender;

用例2

根据性别分组，分别统计男性和女性的平均年龄

select gender as '性别',avg(age) as '平均年龄' from tb_user group by gender;

用例3

查询年龄小于30岁的人员，并按政权分组，获取人员数量大于3的政权

elect county as '政权',count(*) as '人数' from tb_user where age<30 group by county having count(*)>=3;