SQL condition if doesn't exist - mysql

Question

so I have this SQL query:
SELECT
p.ID
FROM
`cdlr_posts` p,
cdlr_postmeta pm
WHERE
pm.post_id=p.ID AND
`post_type` = 'shop_order' AND
pm.meta_key = '_statusCDLR' AND
pm.meta_value <> 1
group by
p.ID
What I need is to show all the IDS if they match with those conditions, but I will also like to show the ones that do not contain the "_statusCDLR" meta_key I tried something like this with no luck:
WHERE
pm.post_id=p.ID AND
`post_type` = 'shop_order' AND
(pm.meta_key = '_statusCDLR' AND pm.meta_value <> 1 OR pm.meta_key <> '_statusCDLR')
group by
Any help will be appreciated to achieve what I need.

Answer 1

I understand that your requirement is to select the id of posts that :
either have a corresponding record in cdlr_post_meta with meta_key = 'statusCDLR' and meta_value <> 1
or do not have a record cdlr_post_meta with meta_key = 'statusCDLR'
A strategy to achieve this is to use a LEFT JOIN to search for a record in cdlr_post_meta with meta_key = 'statusCDLR', and then implement the rest of the logic in the WHERE clause (if there is no corresponding record, the columns of pm are all NULL).
SELECT p.ID
FROM cdlr_posts p
LEFT JOIN cdlr_postmeta pm
ON pm.post_id = p.ID AND pm.meta_key = '_statusCDLR'
WHERE
p.post_type = 'shop_order'
AND ( pm.post_id IS NULL OR pm.meta_value <> 1 )
GROUP BY p.ID
PS - General remarks regarding your sql :
When mixing ORs and ANDs, you need to surround the test expressions within parentheses to avoid running into prescedence issues (AND has higher precedence than OR).
you should use explicit JOINs instead of implicit ones.