* @since    3.15.0
	 * @version  3.15.0
	 */
	protected function sql_where() {

		global $wpdb;

		$sql = 'WHERE 1';

		foreach ( array( 'post_id', 'user_id' ) as $key ) {

			$ids = $this->get( $key );
			if ( $ids ) {
				$prepared = implode( ',', $ids );
				$sql     .= " AND {$key} IN ({$prepared})";
			}
		}

		if ( $this->get( 'query' ) ) {

			$sql .= ' AND ( ';

			foreach ( $this->get( 'query' ) as $i => $query ) {

				if ( 0 !== $i ) {
					$sql .= " {$this->get( 'query_compare' )} ";
				}

				// phpcs:disable WordPress.DB.PreparedSQL.InterpolatedNotPrepared

				switch ( $query['compare'] ) {

					case '=':
					case '!=':
					case 'LIKE':
						$sql .= $wpdb->prepare( "( meta_key = %s AND meta_value {$query['compare']} %s )", $query['key'], $query['value'] );
						break;

					case 'IN':
					case 'NOT IN':
						$query['value'] = array_map( array( $this, 'escape_and_quote_string' ), $query['value'] );
						$vals           = implode( ',', $query['value'] );
						$sql           .= $wpdb->prepare( "( meta_key = %s AND meta_value {$query['compare']} ( {$vals} ) )", $query['key'] );
						break;

					case 'IS NOT NULL':
						$sql .= $wpdb->prepare( "( meta_key = %s AND meta_value {$query['compare']} )", $query['key'] );
						break;

				}

				// phpcs:enable WordPress.DB.PreparedSQL.InterpolatedNotPrepared

			}

			$sql .= ' )';

		}