WordPress: Retrieve Parent Categories of a Post Using WP_Query

To get the categories of a post in WordPress using WP_Query, you can use the get_the_category() function. Here's an example:

$args = array(
    'post_type' => 'post',
    'posts_per_page' => -1 // Retrieve all posts
);

$query = new WP_Query($args);

if ($query->have_posts()) {
    while ($query->have_posts()) {
        $query->the_post();

        $categories = get_the_category();
        
        foreach ($categories as $category) {
            if ($category->parent == 0) {
                echo $category->name . '<br>'; // Output parent category name
            }
        }
    }
    
    wp_reset_postdata();
} else {
    echo 'No posts found.';
}

In the above example, we are using WP_Query to retrieve all posts and then looping through each post using the_post() function. Within the loop, we use get_the_category() to retrieve the categories associated with the current post. We then iterate through the categories and check if the parent property is 0, which indicates a parent category. If it is, we output the category name.

Note that we also use wp_reset_postdata() after the loop to reset the post data to the main query. This is necessary if you are using this code within a template file to prevent any conflicts with other queries on the page.