如何将if语句在Informix中触发

Question

I need to make a trigger that won't allow update of "grade" where grade is 1. So, I need to make it with before the triggering statement. Procedure returnMistake() is not of a concern - it just raises exception .

I have the following code:

CREATE TRIGGER changeGrade
    UPDATE OF grade ON subject
    BEFORE (
        IF (grade = 1) EXECUTE PROCEDURE returnMistake() END IF; 
    )

I know that I don't specify anywhere value of the grade , but just name of the column, but I can't put REFERENCING OLD because I'm using BEFORE and not FOR EACH ROW .

How can I make this? This stops at IF statement describing syntax error. For the matter of example, I'll put here code that is passing in Informix:

CREATE TRIGGER changeGrade
        UPDATE OF grade ON subject
        BEFORE (
            EXECUTE PROCEDURE returnMistake() 
        )

How to do it with BEFORE ? I would know how to make it with FOR EACH ROW - I would just let it to be changed, and then if it was 1, change it back to the previous and calling the returnMistake() , although I think I'm supposed to use BEFORE .

Answer 1

Unfortunately Informix (at current version , v12.10) don't support embedded SPL statements into a trigger "body", which make little annoying include some logical at triggers since you are obligated to create a SPL to do the job...
For more simplistic logical or filter you can use the WHEN option.

That said, your solution don't appear much logical for me... or you are misinterpreted the concept of before, for each row and after actions from a trigger.

What I means is: If you use the before action, the trigger will be called only once before of the UPDATE statement (no matter if this update will affect one, non or lot of rows).
If the before trigger was executed only once and (most important) before the statement start running, how they will able to know the content of the "grade" field for all rows which could be affected?

So, if you want to create a exception if the row with "grade" content equal 1 , you are without options and need use the for each row + when + stored procedure + raise exception ...

If your database is logged (have transaction control) and this table is standard (not a RAW table) the raised exception will abort and rollback all rows affected by this update statement. This way you will reach your objective.

CREATE TRIGGER changeGrade
UPDATE OF grade ON subject
REFERECING OLD AS O
WHEN ( o.grade =1 )   
FOR EACH ROW ( EXECUTE PROCEDURE returnMistake() )