Added db trigger to enforce unique course handels

db_schema.sql

@@ -273,4 +273,5 @@ CREATE VIEW IF NOT EXISTS `lectures` AS select `lectures_data`.* from `lectures_
 CREATE VIEW IF NOT EXISTS `videos` AS select `videos_data`.* from `videos_data` join `lectures_data` on (`lectures_data`.`id` = `lecture_id`) join `courses_data` on (`courses_data`.`id` = `course_id`) where (not(`videos_data`.`deleted` or `lectures_data`.`deleted` or `courses_data`.`deleted`));
 CREATE VIEW IF NOT EXISTS `sorterrorlog` AS select * from `sorterrorlog_data` where (not(`sorterrorlog_data`.`deleted`));
 CREATE TRIGGER IF NOT EXISTS featured_unique_order AFTER INSERT ON featured FOR EACH ROW BEGIN UPDATE featured SET `order` = (SELECT MAX(`order`) FROM featured)+1 WHERE id = NEW.id; END;
+CREATE TRIGGER IF NOT EXISTS courses_unique_handle BEFORE UPDATE OF handle ON courses_data FOR EACH ROW WHEN NEW.handle IN (SELECT handle FROM courses WHERE NOT deleted) BEGIN SELECT RAISE(ROLLBACK, "Handle bereits in Verwendung"); END;
 COMMIT;