[Mysql 查询语句]——分组查询group by

#group by
(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字段的全部值只有两个(‘男‘和‘女‘)，所以分为了两组
当group by单独使用时，只显示出每组的第一条记录
所以group by单独使用时的实际意义不大

#group by + group_concat()
(1) group_concat(字段名)可以作为一个输出字段来使用，
(2) 表示分组之后，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) 通过group_concat()的启发，我们既然可以统计出每个分组的某字段的值的集合，那么我们也可以通过集合函数来对这个"值的集合"做一些操作

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作用和where一样，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的作用是：在最后新增一行，来记录当前列里所有记录的总和

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       |
+------+-------------------+