（转载）MySQL关键字GROUP BY的使用

例子：

mysql> select * from employee;
+------+------+-------+------+-------+----------+
| num  | d_id | name  | age  | sex   | homeaddr |
+------+------+-------+------+-------+----------+
|    1 | 1001 | jack  |   26 | man   | beijing  |
|    2 | 1001 | mike  |   25 | man   | nanjing  |
|    3 | 1003 | alex  |   24 | man   | nanchang |
|    4 | 1004 | rose  |   14 | woman | england  |
|    5 | 1001 | robot |   20 | man   | zhuhai   |
+------+------+-------+------+-------+----------+
5 rows in set (0.00 sec)

// 这里采取按照不同的年龄进行分组，当然也可以按照不同的地区分组
mysql> select * from employee group by sex;
+------+------+------+------+-------+----------+
| num  | d_id | name | age  | sex   | homeaddr |
+------+------+------+------+-------+----------+
|    1 | 1001 | jack |   26 | man   | beijing  |
|    4 | 1004 | rose |   14 | woman | england  |
+------+------+------+------+-------+----------+
2 rows in set (0.00 sec)

mysql>

mysql> select sex, GROUP_CONCAT(name) from employee group by sex;
+-------+----------------------+
| sex   | GROUP_CONCAT(name)   |
+-------+----------------------+
| man   | jack,mike,alex,robot |
| woman | rose                 |
+-------+----------------------+
2 rows in set (0.05 sec)

mysql>

// 统计男、女分组中，每组的人数
mysql> select sex, count(sex) from employee group by sex;
+-------+------------+
| sex   | count(sex) |
+-------+------------+
| man   |          4 |
| woman |          1 |
+-------+------------+
2 rows in set (0.00 sec)

// 找出分组中个数>=的分组
mysql> select sex, count(sex) from employee group by sex HAVING COUNT(sex) >= 3;

+------+------------+
| sex  | count(sex) |
+------+------------+
| man  |          4 |
+------+------------+
1 row in set (0.00 sec)

mysql>


// 根据d_id分组，当相同的分组中sex不同时，在进行分组
mysql> select * from employee group by d_id, sex;
+------+------+------+------+-------+----------+
| num  | d_id | name | age  | sex   | homeaddr |
+------+------+------+------+-------+----------+
|    1 | 1001 | jack |   26 | man   | beijing  |
|    3 | 1003 | alex |   24 | man   | nanchang |
|    4 | 1004 | rose |   14 | woman | england  |
+------+------+------+------+-------+----------+
3 rows in set (0.00 sec)

mysql> select * from employee;
+------+------+-------+------+-------+----------+
| num  | d_id | name  | age  | sex   | homeaddr |
+------+------+-------+------+-------+----------+
|    1 | 1001 | jack  |   26 | man   | beijing  |
|    2 | 1001 | mike  |   25 | man   | nanjing  |
|    3 | 1003 | alex  |   24 | man   | nanchang |
|    4 | 1004 | rose  |   14 | woman | england  |
|    5 | 1001 | robot |   20 | man   | zhuhai   |
+------+------+-------+------+-------+----------+
5 rows in set (0.00 sec)

mysql> insert into employee values(6, 1001, 'nick', 33, 'woman', 'shenzhen');
Query OK, 1 row affected (0.00 sec)

mysql> select * from employee;
+------+------+-------+------+-------+----------+
| num  | d_id | name  | age  | sex   | homeaddr |
+------+------+-------+------+-------+----------+
|    1 | 1001 | jack  |   26 | man   | beijing  |
|    2 | 1001 | mike  |   25 | man   | nanjing  |
|    3 | 1003 | alex  |   24 | man   | nanchang |
|    4 | 1004 | rose  |   14 | woman | england  |
|    5 | 1001 | robot |   20 | man   | zhuhai   |
|    6 | 1001 | nick  |   33 | woman | shenzhen |
+------+------+-------+------+-------+----------+
6 rows in set (0.00 sec)

// 从这里看到，当执行d_id分组时，若遇到相同的值，
则进行sex分组。
mysql> select * from employee group by d_id, sex;
+------+------+------+------+-------+----------+
| num  | d_id | name | age  | sex   | homeaddr |
+------+------+------+------+-------+----------+
|    1 | 1001 | jack |   26 | man   | beijing  |
|    6 | 1001 | nick |   33 | woman | shenzhen |
|    3 | 1003 | alex |   24 | man   | nanchang |
|    4 | 1004 | rose |   14 | woman | england  |
+------+------+------+------+-------+----------+
4 rows in set (0.00 sec)

mysql>

// 将所有分组的综合相加
mysql> select sex, COUNT(sex) from employee group by sex with rollup;
+-------+------------+
| sex   | COUNT(sex) |
+-------+------------+
| man   |          4 |
| woman |          2 |
| NULL  |          6 |
+-------+------------+
3 rows in set (0.00 sec)

mysql>