mysql命令行 - 无重复查询

这一节我们要学习：

如何写无重复查询语句？

无重复查询即独有查询，也就是查询满足条件的、但又互不相同的结果。首先让我们看一个例子：

select title from employee_data;
+----------------------------+
| title                      |
+----------------------------+
| CEO                        |
| Senior Programmer          |
| Senior Programmer          |
| Web Designer               |
| Web Designer               |
| Programmer                 |
| Programmer                 |
| Programmer                 |
| Programmer                 |
| Multimedia Programmer      |
| Multimedia Programmer      |
...

上述例子里，我们在employee_data表里查询title(职务)，注意到结果里有许多是重复的。如果我们想得到没有重复的结果，就可以用到mysql的关键字 - DISTINCT

select DISTINCT title from employee_data;
+----------------------------+
| title                      |
+----------------------------+
| CEO                        |
| Customer Service Manager   |
| Finance Manager            |
| Marketing Executive        |
| Multimedia Programmer      |
| Programmer                 |
| Senior Marketing Executive |
| Senior Programmer          |
| Senior Web Designer        |
| System Administrator       |
| Web Designer               |
+----------------------------+
11 rows in set (0.00 sec)

这样就可以把公司的所有职务类别无重复地列出。另外，我们还可以同时使用DISTINCT和ORDBER BY把独有的(无重复的)结果排序列出，例如：

select DISTINCT age from employee_data
ORDER BY age; 
+------+
| age  |
+------+
|   25 |
|   26 |
|   27 |
|   28 |
|   30 |
|   31 |
|   32 |
|   33 |
|   34 |
|   35 |
|   36 |
|   43 |
+------+
12 rows in set (0.00 sec)