Question

我无法理解这一点。

我有一个名为1_related的表

id,prod_pack,fk_prod_pack_id,fk_prod_pack_related,fk_prod_pack_related_id
'2', '0', '102', '0', '2482'
'3', '0', '102', '0', '3147'
'2294', '0', '102', '1', '10'

我还有另外两个名为1_products和1_packages的表

我想根据fk_prod_pack_related的值加入任一表的内容（0表示1_products表，1表示1_packages表）

此查询从1_related返回第2行和第3行，并加入1_products

的内容

SELECT 
    r . *, p . *
FROM
    1_related AS r
        INNER JOIN
    1_products as p ON r.fk_prod_pack_related_id = p.id
        and r.fk_prod_pack_related = 0
WHERE
    r.prod_pack = 0
        and r.fk_prod_pack_id = 102

但是我也需要来自1_packages的数据，所以我尝试了这个，但它没有返回任何内容

SELECT 
    r . *, p . *, s . *
FROM
    1_related AS r
        INNER JOIN
    1_products as p ON r.fk_prod_pack_related_id = p.id
        and r.fk_prod_pack_related = 0
        INNER JOIN
    1_packages as s ON r.fk_prod_pack_related_id = s.id
        and r.fk_prod_pack_related = 1
WHERE
    r.prod_pack = 0
        and r.fk_prod_pack_id = 102

Answer 1

您需要左外连接，因为只有一个查询匹配：

SELECT 
    r . *, p . *, s . *
FROM
    1_related AS r
        left outer JOIN
    1_products as p ON r.fk_prod_pack_related_id = p.id
        and r.fk_prod_pack_related = 0
        left outer JOIN
    1_packages as s ON r.fk_prod_pack_related_id = s.id
        and r.fk_prod_pack_related = 1
WHERE
    r.prod_pack = 0
        and r.fk_prod_pack_id = 102

从2个表中选择，取决于第一个表中的2列

1 个答案: