# AVG()

AVG() 函数返回数值列的平均值。

## 1. 实例

查询课程表`courses`，计算表中学生上课人数`student_count`的平均值

```sql
SELECT AVG(student_count) AS average_student_count
FROM courses;
```

执行输出结果

```bash
mysql> SELECT AVG(student_count) AS average_student_count
    -> FROM courses;
+---------------------------+
| average_student_count |
+---------------------------+
|                  929.0000 |
+---------------------------+
1 row in set (0.00 sec)
```

其中`AS`相当于给`AVG(student_count)`取了个显示在列表中的别名

### 练习题：AVG

题目描述：查询教师表`teachers`中教师邮箱为’qq.com‘结尾的年龄的平均值

```sql
SELECT AVG(age)
FROM teachers
WHERE email LIKE '%@qq.com';
```

目标输出结果

```bash
mysql> SELECT AVG(age)
    -> FROM teachers
    -> WHERE email LIKE '%@qq.com';
+----------+
| AVG(age) |
+----------+
|  21.0000 |
+----------+
1 row in set (0.00 sec)
```