SQL - SUM 函数

SQL SUM 函数用于找出各种记录中某个字段的总和。

为了理解SUM 函数，请考虑一个employee_tbl 表格，其中包含以下记录：

SQL> 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)

现在假设根据上表，您想计算所有 dialy_typing_pages 的总和，那么您可以使用以下命令：

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

您可以使用GROUP BY 子句对各种记录集求和。以下示例将对与单个人员相关的所有记录求和，您将获得每个人键入的页面总数。

SQL> 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)