Question

修订。我的SQL有限，所以非常感谢您的帮助。在一次点击中，我想通过ID（$ ga_id）查询两个表，每个表都有一个内连接到另一个表（表有完全相同的列，父表彼此没有关系）。

我可以成功查询net_5_postmeta表，如下所示

SELECT p.*
FROM net_5_postmeta AS pm
INNER JOIN net_5_posts AS p ON pm.meta_value=p.ID 
WHERE pm.post_id = $ga_id
AND pm.meta_key = '_thumbnail_id' 
ORDER BY p.post_date DESC 
LIMIT 4

但是我想查询net_5_postmeta表和net_7_postmeta表（它们具有相同的列），我认为它需要是... ...

SELECT p.*
FROM net_5_postmeta, net_7_postmeta AS pm
INNER JOIN net_5_posts AS p ON pm.meta_value=p.ID
INNER JOIN net_7_posts AS p ON pm.meta_value=p.ID 
WHERE pm.post_id = $ga_id
AND pm.meta_key = '_thumbnail_id' 
ORDER BY p.post_date DESC 
LIMIT 4

Answer 1

正如你所说父表没有任何关系我只能假设你想要将 5 和 7 表视为相同。您应该使用union all将数据合并在一起，然后执行ORDER BY / LIMIT。

SELECT *
FROM
(
  SELECT p.*
  FROM net_5_postmeta AS pm
  INNER JOIN net_5_posts AS p ON pm.meta_value=p.ID 
  WHERE pm.post_id = $ga_id
  AND pm.meta_key = '_thumbnail_id' 

  UNION ALL

  SELECT p.*
  FROM net_7_postmeta AS pm
  INNER JOIN net_7_posts AS p ON pm.meta_value=p.ID 
  WHERE pm.post_id = $ga_id
  AND pm.meta_key = '_thumbnail_id' 
) united
ORDER BY united.post_date DESC 
LIMIT 4

SQL查询多个联接

1 个答案: