Custom SELECT Query With Multiple Meta Values (Ordering Problem)

Question

I have two custom meta values that I am using for an events list and have almost got everything working perfectly except for the ordering. I have two dates (event_date and event_enddate). One of these is a start date which is always present, and the other is an end date which is only used when needed. I have created a query which lists all dates where either of these fields are equal or greater to today's date, which works fine. However, I want all of the events to be ordered solely by the event_date field. How can I do this? I've spent all day trying to do this and just can't work it out!

Here's my code:

SELECT DISTINCT wposts.*
FROM $wpdb->posts wposts
LEFT JOIN $wpdb->postmeta wpostmeta ON wposts.ID = wpostmeta.post_id
LEFT JOIN $wpdb->term_relationships ON (wposts.ID = $wpdb->term_relationships.object_id)
LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
WHERE ((wpostmeta.meta_key = 'event_enddate' AND wpostmeta.meta_value >= CURDATE())
 OR (wpostmeta.meta_key = 'event_date' AND wpostmeta.meta_value >= CURDATE()))
AND wposts.post_status = 'publish'
AND $wpdb->term_taxonomy.taxonomy = 'category'
AND $wpdb->term_taxonomy.term_id IN(15)
AND wposts.post_status = 'publish'
ORDER BY wpostmeta.meta_value ASC

Answer 1

I believe you can achieve this with WP_Query and avoid the custom SQL:

$today = date( 'Y-m-d' );
$args = array(
    'tax_query' => array(
        array(
            'taxonomy' => 'category',
            'field' => 'ID',
            'terms' => 15
        )
    ),
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key' => 'event_enddate',
            'value' => $today,
            'compare' => '>='
        )
    ),
    'meta_key' => 'event_date',
    'meta_value' => $today,
    'meta_compare' => '>=',
    'orderby' => 'meta_value_num'
);
$events = new WP_Query( $args );