1

I need to get all posts that do not have a particular custom field, (e.g. custom_field).

The following query works fine:

SELECT * FROM wp_posts WHERE ID NOT IN (SELECT post_id FROM wp_postmeta WHERE meta_key = "custom_field")

I try to avoid using subqueries if possible. How can this query be rewritten so it doesn't use a subquery? I tried writing it with a JOIN, but I don't know how to avoid joining the rows for post X entirely if a particular meta_key custom field is present.

For example, a naive attempt might be:

SELECT p.* FROM wp_posts p JOIN wp_postmeta pm ON p.id = pm.post_id AND pm.meta_key != "custom_field"

However, this query simply avoids joining on rows with a meta_key matching "custom_field". Other rows for that post will still be joined. We wish to get all posts that don't have a custom field with a meta_key of "custom_field".

Any thoughts?

2

This is the way WordPress does it:

SELECT wp_posts.* FROM wp_posts
LEFT JOIN wp_postmeta ON ( wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = 'custom_field' )
WHERE wp_postmeta.post_id IS NULL

You can do this with WP_Query meta_query and 'compare' => 'NOT EXISTS'.

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