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3.3.4.2 选择特定行

如前一节所示,检索整个表很容易。只需从语句中省略该WHERE子句SELECT即可。但通常您不想看到整个表,尤其是当它变大时。相反,您通常对回答特定问题更感兴趣,在这种情况下,您可以对所需信息指定一些限制条件。让我们看看他们回答的关于您的宠物的一些选择查询。

您只能从表中选择特定行。例如,如果您想验证您对 Bowser 的出生日期所做的更改,请像这样选择 Bowser 的记录:

mysql> SELECT * FROM pet WHERE name = 'Bowser';
+--------+-------+---------+------+------------+------------+
| name   | owner | species | sex  | birth      | death      |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+

输出确认年份被正确记录为 1989,而不是 1979。

字符串比较通常不区分大小写,因此您可以将名称指定为'bowser''BOWSER'等。查询结果是一样的。

您可以在任何列上指定条件,而不仅仅是 name. 例如,如果您想知道哪些动物是在 1998 年期间或之后出生的,请测试该 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  |
+----------+-------+---------+------+------------+-------+

您可以组合条件,例如,定位母狗:

mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f';
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

前面的查询使用AND 逻辑运算符。还有一个 OR运算符:

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并且 OR可以混合使用,尽管 AND优先级高于 OR. 如果您同时使用这两种运算符,最好使用括号来明确指示应如何对条件进行分组:

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  |
+-------+--------+---------+------+------------+-------+