mysql - SQL查询多重连接

Question

已修订。我的 SQL 有限，非常感谢您的帮助。在一次命中中，我想通过 ID ($ga_id) 查询两个表，每个表都与另一个表有内部联接(表具有完全相同的列，并且父表彼此没有关系)。

我可以成功查询net_5_postmeta表，如下

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)，我认为它需要类似于...

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

正如您所说，父表没有关系，我只能假设您希望将 5 和 7 表视为相同的表。您应该使用 union all 将它们的数据组合在一起，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