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联结

SQL最强大的功能之一就是能在数据检索查询的执行中联结表。而联结表是基于关系表。理解关系表如下,假如由同一供应商生产的多种物品,在数据库中需要存储产品信息,而在产品信息的这一行不存储供应商信息,因为这些产品的供应商是同一个,所以在产品信息这一行里记录一个唯一的供应商id就可以,供应商信息单独的存储一个表,并且也用为一个的id记录。这样,产品表与供应商表就通过 共同的供应商id连接起来了。如下表所示:


在vendors表中,vend_id称为主键,products表中的vend_id为外键。
为什么要用联结表呢,将数据分解为多个表能够更有效的存储,更方便的处理,并且具有更大的可伸缩性。但是这些好处是有代价的。如果数据存储在多个表中,怎么样用单条SELECT语句检索出数据?答案是使用联结。简单的说联结是一种机制,用来在一条SELECT语句中关联表,因此称之为联结。
下面的示例最主要的地方就是 vendors.vend_id = products.vend_id, 在比较时是 从 vendors表中逐一拿出每一行,这个每一行与products表中的所有行进行比较,筛选出products表中的满足条件的每一行。


此外联结表还可以用 INNER JOIN .. ON来表示,语句的作用与上面的示例是一样的:


联结多个表, 可以如下理解下例,WHERE的三个条件,第一个条件,产品表中的每一个产品都会有一个vend_id,所有这个条件不会过滤掉一些产品;第二个条件,处于订单上的产品,应该是所有产品的一部分分;第三个条件,订单编号为20005的产品;

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