我可以阻止Oracle 11g上的触发器内的插入吗？

Question

So, what I want to do is to block an insert if the condition is met. In this case, I'm developing a simple library database system and I don't want to allow someone to be able to borrow a book if that certain person has already borrowed 2 books and still hasn't returned any of them.

I need to do this via trigger, is it possible? Also, I'd like to know if I can supply a query into the WHEN clause like I did.

CREATE OR REPLACE TRIGGER trigger_borrowing_limit
BEFORE INSERT ON Borrowing
REFERENCING NEW ROW AS NROW
FOR EACH ROW
WHEN SELECT COUNT(*) FROM Borrowing WHERE RETURN_DATE IS NULL AND NROW.ID = Borrowing.ID > 2
BEGIN
?
END

Answer 1

You can't supply a query for the WHEN clause. From the Oracle 11 docs (search for "WHEN (condition)" once you get to the page):

Restrictions on WHEN (condition)

• If you specify this clause, then you must also specify FOR EACH ROW.

• The condition cannot include a subquery or a PL/SQL expression (for example, an invocation of a user-defined function).

As for preventing the row insert, the comment from Mihai is the way to go. Your front end can catch the exception, and if it's error number -20101 (per Mihai's example) you'll know that the person already has two books out. Note that some drivers will report the absolute value of the exception number, so the error number that reaches you may be 20101 .

---

Addendum: a followup question asked how to apply the "two books out" logic since it's not valid for the WHEN clause.

The answer is to drop the WHEN clause and put the logic into the trigger body. Note that I normally stick with the standard NEW for referencing the new row, so my answer doesn't have the REFERENCING NEW AS NROW :

CREATE OR REPLACE TRIGGER trigger_borrowing_limit
  BEFORE INSERT ON Borrowing
DECLARE
  booksOut NUMBER;
BEGIN
  SELECT COUNT(*) INTO booksOut
    FROM Borrowing
    WHERE RETURN_DATE IS NULL AND NEW.ID = Borrowing.ID;
  IF booksOut > 2 THEN
    -- Next line courtesy of Mihai's comment under the question
    raise_application_error(-20101, 'You already borrowed 2 books');
  END IF;
END;