🔥group by应用场景🔥
我们已经掌握使用select语句结合where查询条件获取需要的数据，但在实际的应用中，还会遇到下面这类需求，又该如何解决？
1、公司想知道每个部门有多少员工
2、班主任想统计各科成绩的第一名
3、某门店想掌握男、女性会员的人数和平均年龄
🔥group by的使用🔥
从字面上理解，group by表示根据某种规则对数据进行分组，它必须配合聚合函数进行使用，对数据进行分组后可以进行count、sum、avg、max和min等运算。
group by语法
SELECT column_name,aggregate_function(column_name) FROM table_name GROUP BY column_name
说明：
1、aggregate_function 表示聚合函数
2、group by 可以对一列或多列进行分组
mysql> alter table employee add column dept varchar(20) comment '部门名称';
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc employee;
+--------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+----------------+
| id | int | NO | PRI | NULL | auto_increment |
| name | varchar(30) | YES | | NULL | |
| sex | varchar(1) | YES | | NULL | |
| salary | int | YES | | NULL | |
| dept | varchar(20) | YES | | NULL | |
+--------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> update employee set dept='部门A' where id='1';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update employee set dept='部门B' where id='2';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update employee set dept='部门B' where id='3';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from employee;
+----+--------+------+--------+-------+
| id | name | sex | salary | dept |
+----+--------+------+--------+-------+
| 1 | 张三 | 男 | 5500 | 部门A |
| 2 | 李四 | 男 | 4500 | 部门B |
| 3 | 张小妹 | 女 | 4500 | 部门B |
+----+--------+------+--------+-------+
3 rows in set (0.00 sec)select sex,count(*) from employee group by sex;
+------+----------+
| sex | count(*) |
+------+----------+
| 男 | 2 |
| 女 | 1 |
+------+----------+
2 rows in set (0.00 sec)select dept,count(*) from employee group by dept;
+-------+----------+
| dept | count(*) |
+-------+----------+
| 部门A | 1 |
| 部门B | 2 |
+-------+----------+
2 rows in set (0.00 sec)select dept,sum(salary) from employee group by dept;
+-------+-------------+
| dept | sum(salary) |
+-------+-------------+
| 部门A | 5500 |
| 部门B | 9000 |
+-------+-------------+
2 rows in set (0.00 sec)select dept,sum(salary) from employee group by dept;
+-------+-------------+
| dept | sum(salary) |
+-------+-------------+
| 部门A | 5500 |
| 部门B | 9000 |
+-------+-------------+
2 rows in set (0.00 sec)select dept,min(salary) from employee group by dept;
+-------+-------------+
| dept | min(salary) |
+-------+-------------+
| 部门A | 5500 |
| 部门B | 4500 |
+-------+-------------+
2 rows in set (0.00 sec)
🔥 having的使用 🔥
在SQL中增加HAVING子句原因是，WHERE 关键字无法与聚合函数一起使用。HAVING子句可以对分组后的各数据进行筛选。
having语法
SELECT column_name，aggregate_function(column_name)
FROM table_name WHERE column_name operator value
GROUP BY column_name HAVING aggregate_function(column_name) operator valuemysql> select dept, count(*) from employee group by dept;
+-------+----------+
| dept | count(*) |
+-------+----------+
| 部门A | 1 |
| 部门B | 2 |
+-------+----------+
2 rows in set (0.00 sec)
mysql> select dept, count(*) from employee group by dept having count(*)<2;
+-------+----------+
| dept | count(*) |
+-------+----------+
| 部门A | 1 |
+-------+----------+
1 row in set (0.00 sec)
mysql> select dept, count(*) from employee group by dept having count(*)<3;
+-------+----------+
| dept | count(*) |
+-------+----------+
| 部门A | 1 |
| 部门B | 2 |
+-------+----------+
2 rows in set (0.00 sec)mysql> select dept, max(salary) from employee group by dept;
+-------+-------------+
| dept | max(salary) |
+-------+-------------+
| 部门A | 5500 |
| 部门B | 4500 |
+-------+-------------+
2 rows in set (0.00 sec)
mysql> select dept,max(salary) from employee group by dept having max(salary)>=5000;
+-------+-------------+
| dept | max(salary) |
+-------+-------------+
| 部门A | 5500 |
+-------+-------------+
1 row in set (0.00 sec)
