当我们将MySQLSUM()
函数与GROUP BY子句一起使用时,该SUM()
函数将对GROUP BY子句中指定的每个组求和。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)