Question

有没有办法从两个表创建一个视图，其中一个列在两个表中是不同的？我目前遇到的问题是MYSQL告诉我有一个undefined index - 这是完全合理的，因为在一半的情况下，该列将不存在。

表格布局：

(post_rank_activity)
ID, post_id, ... date

(reply_rank_activity)
ID, rank_id, ... date

我想要的结果视图如下：

ID | Post_id | Reply_id | Date
x       x         NULL      x
x      NULL         x        x

和SQL：

$rankView = "Create or replace view userRank as (
select PRA.id, PRA.post_id, PRA.user_id, PRA.vote_up, PRA.rank_date
From post_rank_activity PRA)
union All
(select RRA.id, RRA.reply_id, RRA.user_id, RRA.vote_up, RRA.rank_date
from reply_rank_activity RRA)";

我得到的结果，而不是返回null，它为“reply_id”字段返回“post_id”的值，然后将所有其他值移回 - 见下文：

ID | Post_id      | Reply_id     | Date
x       x             date val      x
x       reply val     date val      x

有什么想法吗？

Answer 1

联合必须在所有部分中以相同的顺序包含相同的列。您应该在联合的每个部分中显式选择/声明空列：

SELECT PRA.id, PRA.post_id, NULL AS reply_id, PRA.user_id, PRA.vote_up, PRA.rank_date
FROM post_rank_activity PRA
UNION All
SELECT RRA.id, NULL AS post_id, RRA.reply_id, RRA.user_id, RRA.vote_up, RRA.rank_date
FROM reply_rank_activity RRA

Answer 2

您的查询应该是

select PRA.id, PRA.post_id, null as Reply_id PRA.rank_date
From post_rank_activity PRA
union All
select RRA.id, null as post_id, RRA.reply_id, RRA.rank_date
from reply_rank_activity RRA

从具有不同列名的两个表创建视图

2 个答案: