[WordPress] WordPress Queries with IN Clauses

00-keys-to-map.php

<?php

// This is used to maintain a map of data should we need to add more.
$data_types = [
	'data_item_one',
	'data_item_two',
	'...'
	'data_item_ten,
];

01-mysql-ready-string.php

<?php

/**
 * Converts the incoming array into a comma-delimited string with
 * quotes wrapped around each key.
 *
 * @access private
 *
 * @param  array $arr The array to convert to a string.
 * @return string     The string representation of the array delimited by quotes and commas.
 */
private function convert_to_sql_ready_string( $arr ) {
  return '"' . implode( $arr, '","' ) . '"';
}

02-the-query.php

<?php

public function get_data_values() {

  global $wpdb;
  $query = "
    SELECT post_id, meta_key, meta_value 
    FROM $wpdb->postmeta WHERE
    meta_key in ( $this->data_types )
    AND 
    meta_value <> '';
  ";

  $results = $wpdb->get_results( $query );

  return $results;
}