http://www.w3school.com.cn/sql/sql_distinct.asp
SQL SELECT DISTINCT
关键词DISTINCT用于返回唯一不同的值。
语法：
SELECT DISTINCT column_name from table_name
实例：
"Orders"表
| Company | OrderNumber |
|---|---|
| IBM | 3532 |
| W3School | 2356 |
| Apple | 4698 |
| W3School | 6953 |
select Company from Orders
返回结果：
| Company |
|---|
| IBM |
| W3School |
| Apple |
| W3School |
select distinct Company from Orders
返回结果：
| Company |
|---|
| IBM |
| W3School |
| Apple |