【转】 [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       |
+------+-------------------+