将MySQL SUM()函数与GROUP BY子句一起使用有什么好处？
当我们将MySQLSUM()函数与GROUPBY子句一起使用时，该SUM()函数将对GROUPBY子句中指定的每个组求和。SUM()与GROUPBY子句一起使用的好处是，我们可以轻松找到特定组的总数。为了理解上述概念，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)
下面的查询将汇总与一个人相关的所有记录，并且每个人的总打字页面总数。
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)