How to find if a post with custom_field == X exists?

Question

I need to find if a post with a custom field X equal to Y exists in a wordpress installation.

Should I do it with a simple sql query or is there something build in that can help me achieve it?

Answer 1

You can use the WP_Query(); to check that like this:

$my_query = new WP_Query();
$my_query->query(array( 'meta_key' => 'X', 'meta_value' => 'Y'));
if ( $my_query->have_posts() ){

     //it exists

} else {
    //it's not here
}

Hope this helps.

Answer 2

The current way would be something like this:

$posts = get_posts(array(
    'meta_key' => 'color',
    'meta_value' => 'blue',
));

if( !empty($posts) ) { // some posts found }

Just note that in upcoming 3.1 querying for custom fields is considerably improved and arguments change, see Custom Field Parameters in Codex.

Answer 3

If you want to use this externally(per your comments), to keep the operation cheap you could do something like this.

<?php
// You'll need to make sure the path is correct here
include_once 'wp-config.php';
include_once 'wp-includes/wp-db.php';

// Of course update the following line with a more appropriate query
$result = $wpdb->get_results("SELECT * FROM $wpdb->posts"); 

print '<pre>';
print_r( $result ); // Remove and do whatever you need to do
print '</pre>';
?>

Loading wp-config.php and wp-db.php should be enough to call upon the $wpdb methods to query the DB for whatever data you need.

Let me know if you need an example SQL query to fetch posts based on meta.