MySQL的AVG函数用来查找各种记录的一个字段的平均值。
为了理解AVG函数，考虑有一个employee_tbl表，其具有以下记录：

mysql> SELECT * FROM employee_tbl;
+------+------+------------+--------------------+
| id   | name | work_date  | daily_typing_pages |
+------+------+------------+--------------------+
|    1 | John | 2007-01-24 |                250 |
|    2 | Ram  | 2007-05-27 |                220 |
|    3 | Jack | 2007-05-06 |                170 |
|    3 | Jack | 2007-04-06 |                100 |
|    4 | Jill | 2007-04-06 |                220 |
|    5 | Zara | 2007-06-06 |                300 |
|    5 | Zara | 2007-02-06 |                350 |
+------+------+------------+--------------------+

7 rows in set (0.00 sec)

现在，假设在上述表的基础上计算所有dialy_typing_pages的平均值，那么可以使用下面的命令：

mysql> SELECT AVG(daily_typing_pages)
    -> FROM employee_tbl;
+-------------------------+
| AVG(daily_typing_pages) |
+-------------------------+
|                230.0000 |
+-------------------------+
1 row in set (0.03 sec)

使用 GROUP BY 子句设置各种记录的平均值。下面的例子将涉及到单个人的记录将有每个人的平均值打印出来。

mysql> SELECT name, AVG(daily_typing_pages)
    -> FROM employee_tbl GROUP BY name;
+------+-------------------------+
| name | AVG(daily_typing_pages) |
+------+-------------------------+
| Jack |                135.0000 |
| Jill |                220.0000 |
| John |                250.0000 |
| Ram  |                220.0000 |
| Zara |                325.0000 |
+------+-------------------------+
5 rows in set (0.20 sec)