1 作用：按分组的字段将每个分组看作一个整体。
2 语法：select 分组字段，聚合函数 from table group by 分组字段。
注意：查询的字段只能是分组字段或者聚合函数，在分组后查询其他非分组字段时没有一样的。将表格看成是柴火，分组相当于用一条绳子对表格按字段捆绑成一束一束的，你不能查看到某一束中的某根柴火。
3 where 和 having的区别
限定的时机：where在分组前进行限定，不满足where条件的将不参与分组。having在分组后进行限定，不满足条件的将不进行显示。
是否能接聚合函数：where后不能跟聚合函数，having后跟的是聚合函数或者分组的字段。
4 例子：
4.1 查询非分组字段会报错
mysql> select * from employee;
+----+------+------+--------+--------+--------+
| id | name | age | salary | phone | in_dpt |
+----+------+------+--------+--------+--------+
| 1 | Tom | 26 | 2500 | 119119 | dpt4 |
| 2 | Jack | 24 | 2500 | 120120 | dpt2 |
| 3 | Rose | 22 | 2800 | 114114 | dpt3 |
| 4 | Jim | 35 | 3000 | 100861 | dpt1 |
| 5 | Mary | 21 | 3000 | 100101 | dpt2 |
| 6 | Alex | 26 | 3000 | 123456 | dpt1 |
| 7 | Ken | 27 | 3500 | 654321 | dpt1 |
| 8 | Rick | 24 | 3500 | 987654 | dpt3 |
| 9 | Joe | 31 | 3600 | 110129 | dpt2 |
| 10 | Mike | 23 | 3400 | 110110 | dpt4 |
| 11 | Jobs | NULL | 3600 | 19283 | dpt2 |
| 12 | Tony | NULL | 3400 | 102938 | dpt3 |
+----+------+------+--------+--------+--------+
12 rows in set (0.00 sec)
mysql> select age,in_dpt from employee group by in_dpt;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'mysql_shiyan.employee.age' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select in_dpt from employee group by in_dpt;
+--------+
| in_dpt |
+--------+
| dpt1 |
| dpt2 |
| dpt3 |
| dpt4 |
+--------+
4 rows in set (0.00 sec)
mysql> select age from employee group by in_dpt;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'mysql_shiyan.employee.age' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql>
4.2 按部门分组查询各个部门的人数
ysql> select in_dpt,count(id) from employee group by in_dpt;
+--------+-----------+
| in_dpt | count(id) |
+--------+-----------+
| dpt1 | 3 |
| dpt2 | 4 |
| dpt3 | 3 |
| dpt4 | 2 |
+--------+-----------+
4 rows in set (0.00 sec)
4.3 查询各个部门中年龄大于30/小于30的人数
mysql> select in_dpt,count(age)from employee where age > 30 group by in_dpt;
+--------+------------+
| in_dpt | count(age) |
+--------+------------+
| dpt1 | 1 |
| dpt2 | 1 |
+--------+------------+
2 rows in set (0.01 sec)
mysql> select in_dpt,count(age)from employee where age < 30 group by in_dpt;
+--------+------------+
| in_dpt | count(age) |
+--------+------------+
| dpt1 | 2 |
| dpt2 | 2 |
| dpt3 | 2 |
| dpt4 | 2 |
+--------+------------+
4 rows in set (0.00 sec)
4.4 查询各个部门年龄<25,人数大于的情况，having和where的联合使用
mysql> select in_dpt,count(age)from employee where age < 25 group by in_dpt;
+--------+------------+
| in_dpt | count(age) |
+--------+------------+
| dpt2 | 2 |
| dpt3 | 2 |
| dpt4 | 1 |
+--------+------------+
3 rows in set (0.00 sec)
mysql> select in_dpt,count(age) m from employee where age < 25 group by in_dpt having m >1;
+--------+---+
| in_dpt | m |
+--------+---+
| dpt2 | 2 |
| dpt3 | 2 |
+--------+---+
2 rows in set (0.00 sec)