Question

I have the table ‘meta_data’ with the following fields:

• id
• post_id
• meta_key
• meta_value

I’d like to loop through and display a list of EACH post (post_id) that has an entry for meta_key='abc' but not one for meta_key='def'

Basically, every post that has a meta_key='abc' entry should have a meta_key='def' entry. I want to generate the list so I can add the missing meta_key='def' entries.

Answer 1

To achieve this you should use the LEFT OUTER JOIN operation joining the same table.

SELECT a.*
FROM meta_data a
LEFT OUTER JOIN meta_data b ON a.post_id = b.post_id AND b.meta_value = 'def'
WHERE 
a.meta_value = 'abc'
AND b.post_id IS null