Question

I need to set a constraint that the user is unable to enter any records after he/she has entered 5 records in a single month. Would it be advisable that I write a trigger or procedure for that? Else is that any other ways that I can setup the constraint?

Instead of writing a trigger i have opt to write a procedure for the constraint but how do i check if the procedure is working?

Below is the procedure:

CREATE OR REPLACE PROCEDURE InsertReadingCheck
        (
         newReadNo          In Int,
         newReadValue       In Int,
         newReaderID        In Int,
         newMeterID         In Int  
        )
AS 
  varRowCount               Int;

BEGIN

Select Count(*) INTO varRowCount
From Reading 
WHERE ReaderID = newReaderID
AND Trunc(ReadDate,'mm') = Trunc(Sysdate,'mm');     

IF (varRowCount >= 5) THEN
    BEGIN
        DBMS_OUTPUT.PUT_LINE('*************************************************');
        DBMS_OUTPUT.PUT_LINE('');
        DBMS_OUTPUT.PUT_LINE(' You attempting to enter more than 5 Records ');
        DBMS_OUTPUT.PUT_LINE('');
        DBMS_OUTPUT.PUT_LINE('*************************************************');
        ROLLBACK;
  RETURN;
    END;

ELSIF (varRowCount < 5) THEN
    BEGIN
        INSERT INTO Reading
            VALUES(seqReadNo.NextVal, sysdate, newReadValue,
            newReaderID, newMeterID);
  COMMIT;
END;
END IF;
END; 

Anyone can help me look through

Answer 1

This is the sort of thing that you should avoid putting in a trigger. Especially the ROLLBACK and the COMMIT. This seems extremely dangerous (and I’m not even sure whether it’s possible). You might have other transactions that you wish to commit that you rollback or vice versa.

Also, by putting this in a trigger you are going to get the following error:

ORA-04091: table XXXX is mutating, trigger/function may not see it

There are ways round this but they’re excessive and involve doing something funky in order to get round Oracle’s insistence that you do the correct thing.

This is the perfect opportunity to use a stored procedure to insert data into your table. You can check the number of current records prior to doing the insert meaning that there is no need to do a ROLLBACK.