3.3.4.3 选择指定列
如果你不想看到表中的所有行, 只需要用逗号分隔感兴趣的列. 例如, 如果你想知道你的动物是什么是时候出生的, 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 |
+----------+------------+
要了解谁拥有宠物, 使用以下查询:
mysql> SELECT owner FROM pet;
+--------+
| owner |
+--------+
| Harold |
| Gwen |
| Harold |
| Benny |
| Diane |
| Gwen |
| Gwen |
| Benny |
| Diane |
+--------+
注意, 查询只是从每条记录中检索 owner
列, 其中某些记录不止出现一次. 要最小化输出, 只徐通过添加 DISTINCT
关键词一次检索处每个唯一的输出记录:
mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner |
+--------+
| Benny |
| Diane |
| Gwen |
| Harold |
+--------+
你可以使用 WHERE
子句组合记录选择和列选择. 例如, 仅仅获取猫狗的出生日期, 使用以下查询:
mysql> SELECT name, species, birth FROM pet
WHERE species = 'dog' OR species = 'cat';
+--------+---------+------------+
| name | species | birth |
+--------+---------+------------+
| Fluffy | cat | 1993-02-04 |
| Claws | cat | 1994-03-17 |
| Buffy | dog | 1989-05-13 |
| Fang | dog | 1990-08-27 |
| Bowser | dog | 1989-08-31 |
+--------+---------+------------+