MySQL 的外键与参照完整性: Part 1
转自:http://sofee.cn/blog/2006/08/07/26/
1. 什么是参照完整性?
——————–
参照完整性(完整性约束)是数据库设计中的一个重要概念,当数据库中的一个表与一个或多个表进行关联时都会涉及到参照完整性。比如下面这个例子:
文章分类表 - categories
category_id name
1 SQL Server
2 Oracle
3 PostgreSQL
5 SQLite
文章表 - articles
article_id category_id title
1 1 aa
2 2 bb
3 4 cc
可见以上两个表之间是通过category_id,其中categories表有4条记录,articles表有3条记录。
然而可能因为某种原因我们删掉了categories 表中category_id=4的记录,而articles表却还是有一条category_id=4的记录,很明显,category_id=4的这条记录不应该存在在articles表中,这样会很容易造成数据错乱。
相反,外键关系(Foreign Key relationships)讨论的是父表(categories)与子表(articles)的关系,通过引入外键(Foreign Key)这个概念来保证参照完整性(Referential integrity),将使会数据库变的非常简单。比如,要要做到删除categories表中category_id=4记录的同时删除 articles 表中category_id=4的所有记录,如果没有引入外键的话,我们就必须执行2条SQL语句才行;如果有外键的话,可以很容易的用一条SQL语句就可以达到要求。
2. 使用外键的条件
—————–
MySQL只在v3.23.34版本以后才引入外键的,所以在这之前的版本就别想了:),除此之外,还必须具备以下几个条件:

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