当我们将 MySQL SUM() 函数与 GROUP BY 子句一起使用时，SUM() 函数会计算 GROUP BY 子句中指定的每个组的总和。将 SUM() 与 GROUP BY 子句一起使用的好处是我们可以轻松找到特定组的总计。要理解上述概念，请考虑一个“employee_tbl”表，该表具有以下记录 -
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)以下查询将汇总与一个人相关的所有记录，您将获得每个人输入的总页数。
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)
