ORACLE: Constraint to restrict number of issues an article may be in

Question

Stuck on this question in a revision paper. I've left the other relations out as they're not relevant to the question.

Consider the following relation: Article(arID, title, journal, issue, year, startpage, endpage)

How would I go about making a constraint that makes sure no more than 5 articles may be published in one particular issue?

Would I do something like:

CREATE ASSERTION ArticlesInIssue
CHECK(
(SELECT COUNT(Issue) FROM Article) <= 5);

I think this wouldn't work fully as it wouldn't be counting for the specific issue, right? So would I need a where clause, or am I going in the totally wrong direction here? Thanks, Mo.

Answer 1

write a trigger that fires upon insert. You can write SQL in the trigger to query the table about-to-be-inserted and do that check. Your code will need to explicitly rollback the insert.

I hope you have an index on the issue column!

Answer 2

One possible solution is to add an articleNo NUMBER(1) NOT NULL column and the constraints:

UNIQUE (issue, articleNo) 

and

CHECK (articleNo BETWEEN 1 AND 5)