数据库关联查询说明

原創 GOon_star 2020-02-23 05:33

关联查询包括：内连接 外连接 全连接 交叉连接（即笛卡尔积）

内连接：只连接匹配的行。

外连接

       左外连接：左边表的全部数据和所连接表的符合连接条件的数据。

       右外连接：与左连接相反，连接表的全部数据和左边表符合连接条件的数据。

      全外连接：又称全连接，即两个表的全部数据。

theta连接

     取两个表的不匹配的行。同内连接相反。

笛卡尔积

     即其中一个表同另一个表的所有行进行匹配连接。

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