How to get all post categories without custom post type categories?

Question

I am creating a Blog in wordpress.

I have a list of categories: Technology, Art, fashion, Home, Lifetime, Education, Business, Religion, Design and home, Marketing

In which some of these categories I am using only in Custom Post Type (Technology, Art, Fashion) and others only in Normal Posts (Home, Life, Education, Business, Religion, Design and home, Marketing).

When I created the custom pos type, I used the CPT UI plugin.

Now I need to get the list of categories that are only being used from normal posts to show them on my blog (). I tried to do the following but it returns all categories including CPTs:

$categories = get_categories();
  foreach($categories as $category) {
    echo '<li class="cat-name"  . '>' . $category->name . '</li>';
}

I just need to show the categories: Home, Life, Education, Business, Religion, Design & Home, Marketing.

And exclude those that are being used in CPT. Please Help!

Answer 1

As pointed out by Tom, this could end up being an expensive query, though from my anecdotal evidence from trying this on a rather large database (granted, only a handful of post types), I think you could write a decently sufficient query.

If you're using caching to store the categories, that could also alleviate some of the potential performance issues.

Here's the query I used for testing (but with my own post types):

SELECT DISTINCT
    tt.taxonomy
FROM
    wp_term_taxonomy tt
JOIN
    wp_term_relationships tr ON( tr.term_taxonomy_id = tt.term_taxonomy_id )
WHERE
    tr.object_id IN ( SELECT ID FROM wp_posts WHERE post_type IN ( 'list', 'of', 'normal', 'post', 'types' ) );

Converting this to something more portable for WordPress:

<?php
global $wpdb;

$post_types   = [ 'list', 'of', 'normal', 'post', 'types' ];
$placeholders = array_fill( 0, count( $post_types ), '%s' );

$sql = <<<SQL
SELECT
    tt.taxonomy
FROM
    {$wpdb->term_taxonomy} tt
JOIN
    {$wpdb->term_relationships} tr ON( tr.term_taxonomy_id = tt.term_taxonomy_id )
WHERE
    tr.object_id IN ( SELECT ID FROM {$wpdb->posts} WHERE post_type IN ( {$placeholders} ) );
SQL;

$query   = $wpdb->prepare( $sql, $post_types );
$results = $wpdb->get_col( $query );