MySQL的Sum()函数是用来找出所有记录的一个字段的总和。
要理解SUM()函数，考虑一个employee_tbl表，它有以下记录：

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，假设要在上述表折基础上计算所有dialy_typing_pages字段值的总和，那么可以使用下面的命令：

mysql> SELECT SUM(daily_typing_pages)
    -> FROM employee_tbl;
+-------------------------+
| SUM(daily_typing_pages) |
+-------------------------+
|                    1610 |
+-------------------------+
1 row in set (0.00 sec)

也可以使用GROUP BY子句设置各个记录总和。下面的例子将计算所有涉及到一个人的记录的总和，并打印每个人总和信息。

mysql> SELECT name, SUM(daily_typing_pages)
    -> FROM employee_tbl GROUP BY name;
+------+-------------------------+
| name | SUM(daily_typing_pages) |
+------+-------------------------+
| Jack |                     270 |
| Jill |                     220 |
| John |                     250 |
| Ram  |                     220 |
| Zara |                     650 |
+------+-------------------------+
5 rows in set (0.17 sec)