MySQL的Sum()函數是用來找出所有記錄的一個字段的總和。
要理解SUM()函數,考慮一個employee_tbl表,它有以下記錄:
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)
現在,假設要在上述表折基礎上計算所有dialy_typing_pages字段值的總和,那麼可以使用下面的命令:
mysql> SELECT SUM(daily_typing_pages) -> FROM employee_tbl; +-------------------------+ | SUM(daily_typing_pages) | +-------------------------+ | 1610 | +-------------------------+ 1 row in set (0.00 sec)
也可以使用GROUP BY子句設置各個記錄總和。下面的例子將計算所有涉及到一個人的記錄的總和,並打印每個人總和信息。
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)