3.4.3 选择性待定的列

如果您不希望从表中看到整行，只需命名您感兴趣的列，用逗号分隔。例如，如果您想知道您的动物何时出生，请选择name和birth列：

mysql> SELECT name, birth FROM pet;
+----------+------------+
| name     | birth      |
+----------+------------+
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Buffy    | 1989-05-13 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+

要了解都谁拥有宠物，请使用此查询：

mysql> SELECT owner FROM pet;
+--------+
| owner  |
+--------+
| Harold |
| Gwen   |
| Harold |
| Benny  |
| Diane  |
| Gwen   |
| Gwen   |
| Benny  |
| Diane  |
+--------+

请注意，查询仅从每条记录中检索所有者列，其中一些记录出现不止一次。为了最小化输出，只需添加一个关键字DISTINCT，每次检索出不重复的记录集：

mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner  |
+--------+
| Benny  |
| Diane  |
| Gwen   |
| Harold |
+--------+