数据库外键是什么意思
数据库外键是什么意思?
外键(FK)是用于建立和加强两个表数据之间的链接的一列或多列。通过将保存表中主键值的一列或多列添加到另一个表中,可创建两个表之间的链接。这个列就成为第二个表的外键。
当创建或更改表时可通过定义 FOREIGN KEY 约束来创建外键。
例如,数据库 pubs 中的 titles 表与 publishers 表有链接,因为在书名和出版商之间存在逻辑联系。
titles 表中的 pub_id 列与 publishers 表中的主键列相对应。titles 表中的 pub_id 列是到 publishers 表的外键。
扩展资料:
保持数据一致性,完整性,主要目的是控制存储在外键表中的数据。 使两张表形成关联,外键只能引用外表中的列的值或使用空值。
学号在成绩表(表2)中是主键,在学生表(表1)中是外键。如果不使用外键,表1的学号字段插了一个值(比如20140999999),但是这个值在表2中并没有,这个时候,数据库允许插入,并不会对插入的数据做关系检查。
然而在设置外键的情况下,插入表1学号字段的值必须要求在表1的学号字段能找到。 同时,如果要删除表2的某个学号字段,必须保证表2中没有引用该字段值的列,否则就没法删除。
这就是所谓的保持数据的一致性和完整性。如右图,如果表1还引用表2的某个学号,却把表1中的这个学号删了,表2就不知道这个学号对应的学生是哪个学生。
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