多表操作
前面我们熟悉了数据库和数据库表的基本操作,现在我们再来看看如何操作多个表。
在一个数据库中,可能存在多个表,这些表都是相互关联的。我们继续使用前面的例子。前面建立的表中包含了员工的一些基本信息,如姓名、性别、出生日期、出生地。我们再创建一个表,该表用于描述员工所发表的文章,内容包括作者姓名、文章标题、发表日期。
1、查看第一个表mytable的内容:
mysql> select * from mytable;
+----------+------+------------+-----------+
| name | sex | birth | birthaddr |
+----------+------+------------+-----------+
| abccs|f | 1977-07-07 | china |
| mary |f | 1978-12-12 | usa |
| tom |m | 1970-09-02 | usa |
+----------+------+------------+-----------+
2、创建第二个表title(包括作者、文章标题、发表日期):
mysql> create table title(writer varchar(20) not null,
-> title varchar(40) not null,
-> senddate date);
向该表中填加记录,最后表的内容如下:
mysql> select * from title;
+--------+-------+------------+
| writer | title | senddate |
+--------+-------+------------+
| abccs | a1| 2000-01-23 |
| mary | b1| 1998-03-21 |
| abccs | a2| 2000-12-04 |
| tom| c1| 1992-05-16 |
| tom| c2| 1999-12-12 |
+--------+-------+------------+
5 rows in set (0.00sec)
3、多表查询
现在我们有了两个表: mytable 和 title。利用这两个表我们可以进行组合查询:
例如我们要查询作者abccs的姓名、性别、文章:
mysql> SELECT name,sex,title FROM mytable,title
-> WHERE name=writer AND name='abccs';
+-------+------+-------+
| name | sex | title |
+-------+------+-------+
| abccs | f| a1|
| abccs | f| a2|
+-------+------+-------+
上面例子中,由于作者姓名、性别、文章记录在两个不同表内,因此必须使用组合来进行查询。必须要指定一个表中的记录如何与其它表中的记录进行匹配。
注意:如果第二个表title中的writer列也取名为name(与mytable表中的name列相同)而不是writer时,就必须用mytable.name和title.name表示,以示区别。
再举一个例子,用于查询文章a2的作者、出生地和出生日期:
mysql> select title,writer,birthaddr,birth from mytable,title
-> where mytable.name=title.writer and title='a2';
+-------+--------+-----------+------------+
| title | writer | birthaddr | birth |
+-------+--------+-----------+------------+
| a2| abccs | china | 1977-07-07 |
+-------+--------+-----------+------------+

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