protected function sql_where() {

		global $wpdb;

		$sql = 'WHERE 1';

		foreach ( array( 'quiz_id', 'student_id' ) as $key ) {
			$ids = $this->get( $key );
			if ( $ids ) {
				$prepared = implode( ',', $ids );
				$sql     .= " AND qa.{$key} IN ({$prepared})";
			}
		}

		// Add attempt lookup.
		$val = $this->get( 'attempt' );
		if ( '' !== $val ) {
			$sql .= $wpdb->prepare( ' AND qa.attempt = %d', $val );
		}

		// Add attempt exclude.
		$exclude = $this->get( 'exclude' );
		if ( $exclude ) {
			$prepared = implode( ',', $exclude );
			$sql     .= " AND qa.id NOT IN ({$prepared})";
		}

		$status = $this->get( 'status' );
		if ( $status ) {
			$prepared = implode( ',', array_map( array( $this, 'escape_and_quote_string' ), $status ) );
			$sql     .= " AND qa.status IN ({$prepared})";
		}

		$status_exclude = $this->get( 'status_exclude' );
		if ( $status_exclude ) {
			$prepared = implode( ',', array_map( array( $this, 'escape_and_quote_string' ), $status_exclude ) );
			$sql     .= " AND qa.status NOT IN ({$prepared})";
		}

		$can_be_resumed = $this->get( 'can_be_resumed' );
		if ( '' !== $can_be_resumed ) {
			$sql .= $wpdb->prepare( ' AND qa.can_be_resumed = %d', $can_be_resumed );
		}

		$search = $this->get( 'search' );
		if ( $search ) {
			$search = $wpdb->esc_like( $search );
			$sql   .= $wpdb->prepare(
				' AND (
					u.user_login LIKE %s
					OR u.user_email LIKE %s
					OR u.display_name LIKE %s
					OR um_first.meta_value LIKE %s
					OR um_last.meta_value LIKE %s
					OR p.post_title LIKE %s
				)',
				'%' . $search . '%',
				'%' . $search . '%',
				'%' . $search . '%',
				'%' . $search . '%',
				'%' . $search . '%',
				'%' . $search . '%'
			);
		}

		return apply_filters( $this->get_filter( 'where' ), $sql, $this );
	}