How can I query 3 posts from diferente categories (one of each category)

Question

Is it possible to have one query calling one post from each category?

Example:

one query calling:

one post from category one one post from category two one post from category tree

If this is not possible, do I have to make 3 queries?

Answer 1

You can create your own function to return posts objects from different categories using the get_object_in_term() function with some customization on the sql request. This function has only one filter (order).

I know that you seem to want a one time WP_Query, but it can give you some idea.

In this example, I directly set an array of term_id and the args (you can add the order parameter, loop through the term_ids instead of directly set the first parameter of the custom function (I needed to randomize each category).

    $term_ids = array(15,11,24);
    $taxonomies = 'product_cat';
    $args = array(
        'global_limit'=>25,
        'posts_per_page'=>1
    );
    foreach ($term_ids as $term_id){
        $terms_object = get_objects_in_term_with_limit( $term_id, $taxonomies, $args ) ;
        foreach ($terms_object as $object){
            $post = get_post($object);
            $post_object[] = array('ID'=>$post->ID, 'post_title'=> $post->post_title, 'term_id'=>$term_id);
        }
    }
    echo json_encode($post_object);

    function get_objects_in_term_with_limit( $term_ids, $taxonomies, $args = array() ) {
        global $wpdb;

        if ( ! is_array( $term_ids ) ) {
            $term_ids = array( $term_ids );
        }
        if ( ! is_array( $taxonomies ) ) {
            $taxonomies = array( $taxonomies );
        }
        foreach ( (array) $taxonomies as $taxonomy ) {
            if ( ! taxonomy_exists( $taxonomy ) ) {
                return new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
            }
        }

        $defaults = array( 'order' => 'ASC', 'posts_per_page'=>5, 'global_limit'=>25 );
        $args = wp_parse_args( $args, $defaults );

        $order = ( 'desc' == strtolower( $args['order'] ) ) ? 'DESC' : 'ASC';

        $global_limit = 'LIMIT '. $args['global_limit'];
        $limit = 'BY RAND() LIMIT '. $args['posts_per_page'];

        $term_ids = array_map('intval', $term_ids );

        $taxonomies = "'" . implode( "', '", array_map( 'esc_sql', $taxonomies ) ) . "'";
        $term_ids = "'" . implode( "', '", $term_ids ) . "'";

        $object_ids = $wpdb->get_col("(SELECT tr.object_id FROM $wpdb->term_relationships AS tr 
              INNER JOIN $wpdb->term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id 
              WHERE tt.taxonomy IN ($taxonomies) AND tt.term_id IN ($term_ids) 
              ORDER BY tr.object_id $order $global_limit) 
              ORDER $limit");

        if ( ! $object_ids ){
            return array();
        }
        return $object_ids;
    }

Hope it helps and that saves your time.