通过关联多个表的列字段，SQL连接可实现跨表数据查询。 1、 连接与键 2、 为获取完整数据，常需从多个表中提取信息，此时需执行连接操作。 3、 数据库中的表可通过键建立关联。主键是表中用于唯一标识每一行记录的列，其值在整张表中具有唯一性。

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