Where is the category of a post stored?

Question

I'm returning to WP after some time away, and find I've forgotten what little I knew.

I have some pages on a site that display a post of given category, which was done with a line like: query_posts($query_string . '&cat=2');

followed by: if ( have_posts() ) : while ( have_posts() ) : the_post(); etc. etc. ending with (else) 'Sorry, no posts matched your criteria'

I have defined a category 'Services', and assigned a test post to this category. I can see in wp_terms that the category has the term_id of 2 and name of 'Services', but the post isn't getting picked up.

Where is the post 'told' what its category is? Should I be expecting to see a field for category_id (or cat_id) somewhere? It's certainly not in the wp_posts record.

Or, in other words, where is the query string &cat=2 picked up?

It certainly used to work, but that was a while ago and maybe some updates to WP or PHP now require a change?

Answer 1

wp_terms, wp_term_taxonomy, wp_term_relationships and wp_term_meta these 4 tables powers the WordPress taxonomy world. You'll find the relation between post and term within these 3 tables wp_terms, wp_term_taxonomy, wp_term_relationships. You have to start journey from wp_term_relationships. If you wanna know more about DB schema https://codex.wordpress.org/Database_Description https://code.tutsplus.com/tutorials/understanding-and-working-with-data-in-wordpress--cms-20567

And query_posts() is harmful and bad practice, it overrides the WordPress main query. Better you should use a custom query.

$query = new WP_Query( array(
        'cat' => 2, // category id goes here
    ) );

if ( $query->have_posts() ) :
    while ( $query->have_posts() ) : $query->the_post();
        // do the whatever you wanna do inside the loop
    endwhile;
endif;