Question

我目前正在尝试创建一个SQL语句，允许我从两个表中检索信息，第三个表从每个表和类型中都有post_id。这样我就可以知道post_id来自哪个表格。

ALL_POSTS：

id PRIMARY KEY,
post_id (FOREIGN KEY)
type

电影：

post_id
type
title

MUSIC：

post_id
type
title
band

这是我使用的SQL语句：

SELECT a.*, b.*, c.* FROM ALL_POSTS as a, MOVIES as b, MUSIC as c WHERE (a.post_id = b.id AND a.type = b.type) OR (a.post_id = c.id AND a.type = c.type)

问题在于它检索了很多结果，并且所有结果都来自MUSIC表，并且大部分都是重复的。

由于

Answer 1

您应该在两个表上使用 LEFT JOIN 。这样，它将返回 ALL_POSTS 表中的所有行以及 MOVIES 和 MUSIC 表中的相应详细信息。

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Answer 2

目前，您正在从三个表中生成笛卡尔积。请改用JOIN：

SELECT A.*, B.*, C.* FROM ALL_POSTS AS A
LEFT JOIN MOVIES AS B ON A.post_id = B.ID AND A.type = B.type
LEFT JOIN MUSIC AS c ON A.post_id = C.ID AND A.type = C.type

SQL - 从两个表中获取元素，第三个表包含所有行

2 个答案: