3.3.4.2 选择特定的行
如上一节所示, 检索整个表很容易. 只需要从 SELECT
语句中省略 WHERE
子句. 但通常当它变大时, 不希望看到整个表. 相反, 你通常更感兴趣的是回答特定的问题, 在这种情况下, 你需要对你想要的信息指定一些约束. 让我们看看一些选择查询, 它们会回答关于你的宠物的问题.
你只能从表中选择特定的行. 例如, 如果你想验证对 Bowser 的出生日期所做的更改, 选择 Bowser 记录, 如下所示:
mysql> SELECT * FROM pet WHERE name = 'Bowser';
+--------+-------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+
输出确认了年份被正确的记录为 1989, 而不是 1979.
字符串通常不区分大小写, 因此你可以将名称指定为 'bowser'
, 'BOWSER'
等等. 查询结果是相同的.
你可以在任何列上指定条件, 而不仅仅是 name
. 例如, 如果你想知道那些动物是在 1998 年之后出生的, 测试 birth
列:
mysql> SELECT * FROM pet WHERE birth >= '1998-1-1';
+----------+-------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+-------+
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Puffball | Diane | hamster | f | 1999-03-30 | NULL |
+----------+-------+---------+------+------------+-------+
你可以组合条件, 例如, 寻找母狗:
mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
mysql> SELECT * FROM pet WHERE species = 'snake' OR species = 'bird';
+----------+-------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+-------+
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
+----------+-------+---------+------+------------+-------+
AND
和 OR
可以混合使用, 尽管 AND
的优先级高于 OR
. 如果同时使用这两个运算符, 最好使用括号来明确表示条件应该如何分组:
mysql> SELECT * FROM pet WHERE (species = 'cat' AND sex = 'm')
OR (species = 'dog' AND sex = 'f');
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+