Question

Revised. My SQL is limited, so really appreciate your help. In one hit, I want to query by ID ($ga_id) two tables each with an inner join to another table (tables have exactly the same columns and the parent tables have no relationship to each other).

I can successfully query the net_5_postmeta table as follows

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

but I want to query the net_5_postmeta table and the net_7_postmeta table (they have the same columns), I think it needs to be something along the lines of…

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

As you say that parent tables have no relationship i can only assume that you want to treat 5 and 7 tables as if they were the same. You should use union all to bring their data together and then do 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