mysql group by having 使用实例

注意：使用group by的时候，SELECT子句中的列名必须为分组列。 如下实例必须包括name列名，因为name是作为group by分组的条件。

实例：

我的数据库中有一张员工工作记录表，表中的数据库如下：

<code class="language-sql">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)</code>

需求一：现在我需要查找出每个员工工作了多少天，这时就需要用到group by语句：

<code class="language-sql">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)</code>

需求二：我现在要查找工作天数大于一天的所有员工，这个时候就需要用到group by having语句

<code class="language-sql">mysql> SELECT name, COUNT(*)
    -> FROM   employee_tbl 
    -> GROUP BY name having COUNT(*)>1 ;
+------+----------+
| name | COUNT(*) |
+------+----------+
| Jack |        2 |
| Zara |        2 |
+------+----------+
2 rows in set (0.04 sec)</code>