Mysql分组查询group by语句详解

(1) group by的含义:将查询结果按照1个或多个字段进行分组，字段值相同的为一组
(2) group by可用于单个字段分组，也可用于多个字段分组

select * from employee; +------+------+--------+------+------+-------------+ | num | d_id | name | age | sex | homeaddr | +------+------+--------+------+------+-------------+ | 1 | 1001 | 张三 | 26 | 男 | beijinghdq | | 2 | 1002 | 李四 | 24 | 女 | beijingcpq | | 3 | 1003 | 王五 | 25 | 男 | changshaylq | | 4 | 1004 | Aric | 15 | 男 | England | +------+------+--------+------+------+-------------+ 
select * from employee group by d_id,sex; 
select * from employee group by sex; +------+------+--------+------+------+------------+ | num | d_id | name | age | sex | homeaddr | +------+------+--------+------+------+------------+ | 2 | 1002 | 李四 | 24 | 女 | beijingcpq | | 1 | 1001 | 张三 | 26 | 男 | beijinghdq | +------+------+--------+------+------+------------+ 根据sex字段来分组，sex字段的所有值只有两个('男'和'女')，因此分为了两组 当group by单独使用时，只显示出每组的第一条记录 因此group by单独使用时的实际意义不大

group by + group_concat()
(1) group_concat(字段名)能够做为一个输出字段来使用，
(2) 表示分组以后，根据分组结果，使用group_concat()来放置每一组的某字段的值的集合

select sex from employee group by sex; +------+ | sex | +------+ | 女 | | 男 | +------+ select sex,group_concat(name) from employee group by sex; +------+--------------------+ | sex | group_concat(name) | +------+--------------------+ | 女 | 李四 | | 男 | 张三,王五,Aric | +------+--------------------+ select sex,group_concat(d_id) from employee group by sex; +------+--------------------+ | sex | group_concat(d_id) | +------+--------------------+ | 女 | 1002 | | 男 | 1001,1003,1004 | +------+--------------------+

group by + 集合函数
(1) 经过group_concat()的启发，咱们既然能够统计出每一个分组的某字段的值的集合，那么咱们也能够经过集合函数来对这个"值的集合"作一些操做

select sex,group_concat(age) from employee group by sex; +------+-------------------+ | sex | group_concat(age) | +------+-------------------+ | 女 | 24 | | 男 | 26,25,15 | +------+-------------------+ 
分别统计性别为男/女的人年龄平均值 select sex,avg(age) from employee group by sex; +------+----------+ | sex | avg(age) | +------+----------+ | 女 | 24.0000 | | 男 | 22.0000 | +------+----------+ 
分别统计性别为男/女的人的个数 select sex,count(sex) from employee group by sex; +------+------------+ | sex | count(sex) | +------+------------+ | 女 | 1 | | 男 | 3 | +------+------------+

group by + having
(1) having 条件表达式：用来分组查询后指定一些条件来输出查询结果
(2) having做用和where同样，但having只能用于group by

select sex,count(sex) from employee group by sex having count(sex)>2; +------+------------+ | sex | count(sex) | +------+------------+ | 男 | 3 | +------+------------+

group by + with rollup
(1) with rollup的做用是：在最后新增一行，来记录当前列里全部记录的总和

select sex,count(age) from employee group by sex with rollup; +------+------------+ | sex | count(age) | +------+------------+ | 女 | 1 | | 男 | 3 | | NULL | 4 | +------+------------+ select sex,group_concat(age) from employee group by sex with rollup; +------+-------------------+ | sex | group_concat(age) | +------+-------------------+ | 女 | 24 | | 男 | 26,25,15 | | NULL | 24,26,25,15 | +------+-------------------+