Question

I have the following table.

CLASS_HAS_STUDENTS (
    PER_SSN INTEGER NOT NULL,
    PER_YEAR INTEGER NOT NULL, /*These two are PKs for a student*/
    SCHOOL_CODE INTEGER NOT NULL, /*PK for a school*/
    CLASS_YEAR INTEGER NOT NULL,
    CLASS_NUMBER INTEGER NOT NULL,
    CLASS_TEACHTYPE CHAR(3) NOT NULL, /*These three are PKs for a class*/
    STUDCLASS_STATUS CHAR(1) NOT NULL
        constraint CKC_STUDCLASS_STATUS_CLASS_TI check (StudClass_Status IN ('E', 'Y', 'T', 'P', 'F')),
    STUDCLASS_LISTNUMBER INTEGER NOT NULL,
    STUDCLASS_ROLLNUMBER INTEGER NOT NULL
);

(This code lacks some minor constraints)

Now, I need a way to check that one PER_SSN/PER_YEAR (a person’s PK) can only have one ‘E’ (“Enrolled”) status. I can’t do this with a trigger (given I’m selecting from the same table) and I don’t know if I can do this with a check constraint (can I use COUNT() here?). Any help is appreciated.

Answer 1

You can create a function-based unique index to enforce this sort of thing. You can’t create a constraint as such.

This takes advantage of the fact that Oracle b-tree indexes do not index NULL data so the index will only have entries for the rows where studclass_status is E.

CREATE UNIQUE INDEX idx_one_enrolled
    ON class_has_students( CASE WHEN studclass_status = 'E'
                                THEN per_ssn
                                ELSE null
                            END,
                           CASE WHEN studclass_status = 'E'
                                THEN per_year
                                ELSE null
                            END );