SQL查询多个联接

Question

修改。 我的SQL受限制，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进行内部联接（表具有完全相同的列，父表之间没有关系）。

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），我认为它需要类似...

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

如您所说的父表没有关系，我只能假设您想将5和7表视为相同。 您应该使用union all将其数据汇总在一起，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