左联接可选表

Question

i try to select table pep_posts then left join with pep_postmeta but table pep_postmeta is optional mean if no data in table pep_postmeta , the result still can show.

below is my query but this query only show data if table pep_postmeta has data, this not what i want.

select p.*, pm.* 
from `pep_posts` p 
left join `pep_postmeta` pm 
  on pm.post_id=p.ID 
where p.post_parent='".$current_page_id."' and
     p.post_status='publish' and pm.meta_key='book_image' 

Answer 1

In a left join, if you reference the optional table in the WHERE clause, it will still filter out only the rows that have a value in that table. What you need to do is move the condition on pm to the ON clause where it won't remove rows, just set the values to NULL .

SELECT p.*, pm.* 
FROM `pep_posts` p 
LEFT JOIN `pep_postmeta` pm 
  ON pm.post_id=p.ID and pm.meta_key='book_image' 
WHERE p.post_parent='".$current_page_id."' AND
     p.post_status='publish'

Answer 2

The problem in your WHERE condition

and pm.meta_key='book_image' 

If pep_postmeta have no data, pm.meta_key value in your selection will be NULL So, you should remove this condtion or rewrite it to something like this

and (pm.meta_key='book_image' or pm.meta_key IS NULL)