我的作业要求用EXCEPT改下边的语句
CREATE ASSERTION NoEmptyCourses
CHECK ( NOT EXISTS (
SELECT * FROM Teaching T
WHERE
NOT EXISTS (
SELECT * FROM Transcript R
WHERE T.CrsCode = R.CrsCode
AND T.Semester = R.Semester)
))
我改成
Create Assertion NoEmptyCourses
Check ( Exists (
Select CrsCode, Semester From Teaching
Except
Select CrsCode, Semester From Transcript
))
请问对吗?
CREATE ASSERTION NoEmptyCourses
CHECK ( NOT EXISTS (
SELECT * FROM Teaching T
WHERE
NOT EXISTS (
SELECT * FROM Transcript R
WHERE T.CrsCode = R.CrsCode
AND T.Semester = R.Semester)
))
我改成
Create Assertion NoEmptyCourses
Check ( Exists (
Select CrsCode, Semester From Teaching
Except
Select CrsCode, Semester From Transcript
))
请问对吗?
NOT EXISTS 和 EXCEPT 不可能有相同的结果