MySQL的SUM函数是用来找出记录中各种的字段的总和。
要了解SUM函数考虑EMPLOYEE_TBL表具有以下记录：
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，假设根据上面的表想来计算所有的dialy_typing_pages的总数，那么可以通过使用下面的命令：
mysql> SELECT SUM(daily_typing_pages)
-> FROM employee_tbl;
+-------------------------+
| SUM(daily_typing_pages)
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