Can a single WP_Query get posts from X tag, but if no results, fallback to X category

Question

Is it possible to run a single WP_Query:

$args = array(
    'tag__in' => 5
);
new WP_Query( $args );

but if 0 posts are found, to show from X category?

Is the only way to check for have_posts() and if zero to run a new query with the category? Hopefully not.

Answer 1

Not sure if this is exactly what you'd want to do because there is still another query being run, but you could use get_term_by before you run the query to check and see if there are any posts with that tag. Then modify the $args array depending on what gets returned.

Haven't done any testing, but something like this should work.

$args = array();
$total = get_term_by( 'id', 5, 'post_tag' );

if ( $total->count > 0 ) {
    $args['tag__in'] = 5;
} else {
    $args['cat'] = 1;
} 
new WP_Query( $args );