SQL允许给表取别名，这样做的两个主要理由是：
缩短SQL语句；允许在单条SELECT语句中多次使用同一个表；与之前相对应的我们使用别名来表示一个表：
自联结
问题：假如知道某一厂商生产的一个产品 ID 为 DTNTR有问题，需要从数据库中查找出该厂商生产的其他所有产品。
下面的SELECT语句可以将该vend_id所生产的所有产品查找出来：
现在来看使用联结的相同查询：
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