Query where ANDing slug values not working

Question

I realize this isn't the most elegant sql. I modified this query where the original query only selected from a single category (the slug field of the terms table). I am trying to obtain a result set which returns items having multiple categories (or slugs) as properties.

This does not return rows, even though the items do have both 'arizona' and 'speech-language-pathologist' in their categories list:

SELECT p.ID FROM wp_term_relationships tr 
JOIN wp_posts p ON tr.object_id = p.ID 
JOIN wp_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id 
JOIN wp_terms t ON tt.term_id = t.term_id 
WHERE p.post_parent = 220 AND t.slug = 'arizona' AND t.slug = 'speech-language-pathology' AND p.post_status = 'publish'

However, if I remove one of the AND conditions, this query does produce a result set:

SELECT p.ID FROM wp_term_relationships tr 
JOIN wp_posts p ON tr.object_id = p.ID 
JOIN wp_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id 
JOIN wp_terms t ON tt.term_id = t.term_id 
WHERE p.post_parent = 220 AND t.slug = 'arizona' AND p.post_status = 'publish'

My SQL is a little rusty, I don't see how the t.slug='something' AND t.slug='something-else' is illogical.

Answer 1

You can do this using the WP APIs like this:

$args = [
    'post_type' => 'post',
    'tax_query' => [
        'relation' => 'AND',
        [
            'taxonomy' => 'your_taxonomy_name',
            'field'    => 'slug',
            'terms'    => array( 'arizona', 'speech-language-pathology' ),
        ]
    ],
];
$query = new WP_Query( $args );

Then you can use $query as you would any standard post loop.

If you only want the IDs you can use the fields option of WP_Query, but keep in mind you won't gain all the caching and performance benefits you might have gotten otherwise, especially if you're going to pass the ID into get_post

$args = [
    'post_type' => 'post',
    'tax_query' => [
        'relation' => 'AND',
        [
            'taxonomy' => 'your_taxonomy_name',
            'field'    => 'slug',
            'terms'    => array( 'arizona', 'speech-language-pathology' ),
        ]
    ],
    'fields' => 'ids'
];
$query = new WP_Query( $args );
$posts = $query->get_posts(); // $posts is an array of post IDs