create trigger teacher_check after insert on section for each row BEGIN if count ( select room_number from section natural join teachers where year = new.year and sec_id = new.sec_id and ID = new.ID and semester = new.semester ) > 1 then rollback; end if; END
当teachers发生变化时,同样只需查看该老师在同一时间是否同时出现在两个不同的课室中即可
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create trigger teacher_check after insert on teachers for each row BEGIN if count ( select room_number from section natural join teachers where year = new.year and course_id = new.course_id and ID = new.ID and semester = new.semester ) > 1 then rollback; end if; END
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