apply_filters_ref_array( ‘posts_join’, string $join, WP_Query $query )

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   Steven Lin 5 years ago

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   Example migrated from Codex:

   To include the required tables in the query use the posts_join filter.

   The below example adds a meta field for use in displaying search results.

   // Join for searching metadata
   function AIOThemes_joinPOSTMETA_to_WPQuery($join) {
       global $wp_query, $wpdb;
   
       if (!empty($wp_query->query_vars['s'])) {
           $join .= "LEFT JOIN $wpdb->postmeta ON $wpdb->posts.ID = $wpdb->postmeta.post_id ";
       }
   
       return $join;
   }
   
   add_filter('posts_join', 'AIOThemes_joinPOSTMETA_to_WPQuery');

   And then the specific search:

   function AIO_AlphabeticSearch_WhereString( $where, &$wp_query )
   {
       global $wpdb;
       if(isset($_GET['aioAlphaSearchMode']) && $_GET['aioAlphaSearchMode'] == 1){
   
           $searchAlphabet = esc_sql($_GET['s']); 
   
           $where .= ' AND ' . $wpdb->posts . '.post_title LIKE \''.$searchAlphabet.'%\' ';
   
           // use only if the post meta db table has been joined to the search tables using posts_join filter
           $where .= " AND ($wpdb->postmeta.meta_key = 'JDReview_CustomFields_ReivewOrNewsPostType' AND $wpdb->postmeta.meta_value = 'JDReview_PostType_ReviewPost') ";
           return $where;
       }
   }
   
   add_filter( 'posts_where', 'AIO_AlphabeticSearch_WhereString', 10, 2 );