外键SQL：如何将一个表中的两个属性引用到在另一个表中包含3列的复合主键？

Question

CREATE TABLE SECTION,`
SectionNo int,
Semester CHAR(7),
CourseID CHAR(8),
PRIMARY KEY (SectionNo, Semester, CourseID),
FOREIGN KEY (CourseID) REFERENCES COURSE (CourseID),

);

CREATE TABLE REGISTRATION(
StudentID int,
SectionNo int,
Semester char(7),
PRIMARY KEY (StudentID, SectionNo, Semester),
FOREIGN KEY (SectionNo, Semester) REFERENCES Section (SectionNo, Semester), 

Underline is coming up under "Section". The error says, "There are no primary or candidate keys in the referenced table 'Section' that match the referencing column list in the foreign key." How do I fix this?!

FOREIGN KEY (StudentID) REFERENCES Student (StudentID),

);

Answer 1

Probably

Judging (uh... guessing) from your chosen names, a student ought to register in a certain semester in a certain section of a certain course . So your registration table is probably missing a course column.

There would be a 3-column FK to the 3-column PK. Also the registration PK would be all 4 columns.

But if

If you were recording just a semester and course, that would be an inclusion dependency (IND) from REGISTRATION to COURSE. A foreign key (FK) is an IND to a key. (Well, in SQL it's an IND to a superkey, SQL KEY/UNIQUE.) An IND from columns to others means every source subrow value ahs to appear as a target subrow value. Ie the projection of the source on its IND columns MINUS the projection of the target on its IND columns has to be empty.

That IND means every REGISTRATION semester-course subrow value has to appear as a COURSE subrow value. Which means

NOT EXISTS (
        SELECT Semester,Course FROM REGISTRATION
    EXCEPT SELECT Semester,Course FROM COURSE
)

If you have no EXCEPT/MINUS then there's an idiom with LEFT JOIN.

You wish

You can't easily express this constraint in most SQL DBMSs. (Although the standard describes the functionality.) You usually have to use triggers on updates to check all but a small number of non-trivial constraints in SQL.