简单介绍MySQL中GROUP BY子句的使用
可以使用GROUP BY组值一列,并且如果愿意的话,可以将该列进行计算。使用COUNT,SUM,AVG等功能的分组列。
要了解GROUP BY子句考虑的EMPLOYEE_TBL的的表具有以下记录:
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)
现在,假设根据上表,我们希望计算每一位员工工作的天数。
如果我们将编写一个SQL查询,如下所示,那么我们将得到下面的结果:
mysql> SELECT COUNT(*) FROM employee_tbl; +---------------------------+ | COUNT(*) | +---------------------------+ | 7 | +---------------------------+
但是,这不是我们的目的服务,我们要显示输入的每个人单独的页面总数。这是通过使用聚合函数一起用GROUP BY子句如下:
mysql> SELECT name, COUNT(*) -> FROM employee_tbl -> GROUP BY name; +------+----------+ | name | COUNT(*) | +------+----------+ | Jack | 2 | | Jill | 1 | | John | 1 | | Ram | 1 | | Zara | 2 | +------+----------+ 5 rows in set (0.04 sec)
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