SQL：内部和外部连接之间的差异

假设你加入列上没有重复，这是迄今为止最常见的情况：

• An inner join of A and B gives the result of A intersect B, ie the inner part of a venn diagram intersection.的一个内部联接和B给出了一个相交乙的结果，即一个维恩图交叉口内的一部分。

• An outer join of A and B gives the results of A union B, ie the outer parts of a venn diagram union.的一个外部联接和B赋予工会B的结果，即维恩图联盟的外部零件。

Examples 例子

Suppose you have two Tables, with a single column each, and data as follows:假设你有两个表，用一个单一的列中的每个，数据如下：

AB - - 1 3 2 4 3 5 4 6

Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.注意：（1,2）是一个独特的，（3,4）是常见的，和（5,6）到B是唯一的

inner join 内部联接

An inner join using either of the equivalent queries gives the intersection of the two tables, ie the two rows they have in common.内部联接使用等效的查询给出两个表的交集，即两行，它们的共同点。

select * from a INNER JOIN b on aa = bb; select a.*,b.* from a,b where aa = bb; a | b --+-- 3 | 3 4 | 4

left outer join 左外连接

A left outer join will give all rows in A, plus any common rows in B.左外连接会给中的所有行，加上B中的任何共同行

select * from a LEFT OUTER JOIN b on aa = bb; select a.*,b.* from a,b where aa = bb(+); a | b --+----- 1 | null 2 | null 3 | 3 4 | 4 ; select * from a LEFT OUTER JOIN b on aa = bb; select a.*,b.* from a,b where aa = bb(+); a | b --+----- 1 | null 2 | null 3 | 3 4 | 4

full outer join 完整外部联接

A full outer join will give you the union of A and B, ie All the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.完全外部联接会给你联盟A和B，即所有排在A和B中的所有行，如果事情在一个不具备相应的基准在B，那么B部分是空的，而副反之亦然。

select * from a FULL OUTER JOIN b on aa = bb; a | b -----+----- 1 | null 2 | null 3 |