MySQL GROUP BY 子句

GROUP BY子句

您可以使用GROUP BY对列中的值进行分组，并且，如果需要，可以对该列执行计算。您可以在分组列上使用COUNT，SUM，AVG等函数。要了解GROUP BY子句，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         |
|   5  | Jill | 2007-04-06 |        220         |
|   6  | Zara | 2007-06-06 |        300         |
|   6  | Zara | 2007-02-06 |        350         |
+------+------+------------+--------------------+
7 rows in set (0.00 sec)

现在，假设根据上表我们要计算每个员工工作的天数。如果我们将编写如下的SQL查询，那么将得到以下结果：

mysql> SELECT COUNT(*) FROM employee_tbl;
+---------------------------+
|          COUNT(*)         |
+---------------------------+
|             7             |
+---------------------------+

但这没有达到我们的目的，我们希望显示每个人分别键入的页面总数。这是通过将聚合函数与GROUP BY子句结合使用来完成的，如下所示：

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)

或者在用户id上分组

SELECT id,name, COUNT(*)
   -> FROM   employee_tbl 
   -> GROUP BY id;
+------+------+----------+
|  id  | name | COUNT(*) |
+------+------+----------+
|   3  | Jack |     2    |
|   5  | Jill |     1    |
|   1  | John |     1    |
|   2  | Ram  |     1    |
|   6  | Zara |     2    |
+------+------+----------+
5 rows in set (0.04 sec)

我们将在SUM，AVG等其他函数中看到更多与GROUP BY相关的功能。