ORDER BY clause constrained by a sub-selection in another table - mysql

Question

I have kind of a edge-case while working on a WordPress project. I am using Advanced Custom Fields to store metadata about posts, which is stored in a "postmeta" table (whereas posts are stored in a "post" table; here prefixed by "otca_").
The posts here are events, which feature a mandatory evt_date and may feature a evt_date_fin ("fin" stands for "end" in French, sorry about the lame naming convention).
My goal is to select a range of events, keeping only those featuring a evt_date set in the future OR a evt_date_fin set in the future (in the latter case, no matter the evt_date), while ordering by evt_date, so as to display them in a paginated, sorted way.
This is what I came up with:
$today = date('Ymd');
$perPage = 12;
$offset = $perPage * ($paged-1); // $paged equals 1, 2, … n (the current page)
$querystr = "
SELECT SQL_CALC_FOUND_ROWS * FROM otca_posts AS post
INNER JOIN otca_postmeta AS meta ON (post.ID = meta.post_id)
WHERE post.post_type = 'agenda'
AND post.post_status = 'publish'
AND (
(meta.meta_key = 'evt_date' AND CAST(meta.meta_value AS CHAR) >= '". $today ."')
OR (meta.meta_key = 'evt_date_fin' AND CAST(meta.meta_value AS CHAR) >= '". $today ."')
)
ORDER BY (meta.meta_key = 'evt_date' AND CAST(meta.meta_value AS CHAR)) ASC
LIMIT ". $perPage ." OFFSET ". $offset;
$evts = $wpdb->get_results($querystr);
$total = $wpdb->get_var('SELECT FOUND_ROWS()');
// then looping over the $evts and using $perPage / $total to build the pagination links
So, for a given post stored in otca_post, there are several records in the otca_postmeta table referencing this post (using post_id), which differ by their meta_key / meta_value pairs (one pair for evt_date, another for evt_date_fin). I use them to filter out posts in the query, which works fine.
The ORDER BY clause does not actually works though.
I would like to know how I could make it so that the posts are ordered by evt_date, a piece of information stored in otca_postmeta; evt_date is not a row, but rather a value stored in the row "meta_key", which means I need to perform a sub-selection somehow in order to… order.
Thank you.

Answer 1

I do this fairly often in my WordPress plugins - you need to join the posts table to the postmeta table for each of the meta_key's that you care about, in this case "evt_date" and "evt_date_fin". Once you have joined them you can use them in the WHERE clause to compare to DATE(CURRENT_TIMESTAMP). Using the MySql function STR_TO_DATE() will convert the strings stored in the meta_value to a DATE datatype - this example assumes you use a Y-m-d format.
$perPage = 12;
$offset = $perPage * ($paged-1);
// create the SQL query by joining posts to postmeta
$querystr = <<<SQL
SELECT SQL_CALC_FOUND_ROWS
p.*,
STR_TO_DATE(s.meta_value, '%Y-%m-%d') AS start_date,
STR_TO_DATE(e.meta_value, '%Y-%m-%d') AS end_date
FROM otca_posts AS p
-- join postmeta for the start
JOIN otca_postmeta AS s ON p.ID = s.post_id AND s.meta_key = 'evt_date'
-- join postmeta for the end
JOIN otca_postmeta AS e ON p.ID = e.post_id AND e.meta_key = 'evt_date_fin'
WHERE
-- is the start greater than or equal to today?
STR_TO_DATE(s.meta_value, '%Y-%m-%d') >= DATE(CURRENT_TIMESTAMP) OR
-- or is the end greater than or equal to today?
STR_TO_DATE(e.meta_value, '%Y-%m-%d') >= DATE(CURRENT_TIMESTAMP)
-- order the results by the start, then the end, then the post title
ORDER BY start_date, end_date, p.post_title
-- paginate the results
LIMIT $perPage OFFSET $offset
SQL;
// get posts that match
$evts = $wpdb->get_results( $querystr );
// get the total number of results
$total = $wpdb->get_var( 'SELECT FOUND_ROWS()' );