外联接（outer join)
2018-04-25 本文已影响12人
轻云绿原
外联接
举个粟子
SELECT *
FROM girls RIGHT OUTER JOIN toys
ON girls.toy_id=toys.toy_id;
查询结果
| girl_id | girl | toy_id | toy_id | toy |
|---|---|---|---|---|
| 1 | Jane | 3 | 3 | toy soldiers |
| 2 | Sally | 4 | 4 | harmonica |
| 3 | Cindy | 1 | 1 | hula hoop |
| 4 | Mandy | 1 | 1 | hula hoop |
| NULL | NULL | NULL | 2 | balsa glider |
| NULL | NULL | NULL | 5 | baseball cards |
| NULL | NULL | NULL | 6 | tinker toys |
| NULL | NULL | NULL | 7 | etch-a-sketch |
| NULL | NULL | NULL | 8 | slinky |
外接联是把一张表当主表，另一张表当次表。主表的所有行都会出现在查询结果里(不论符不符合条件)，次表会根据条件来筛选结果 .
条件是用ON,不是用WHERE
区分主,次表.
LEFT OUTER JOIN:左边是主表,
RIGHT OUTER JOIN:右边是主表.
