Newbie Trigger question - before delete

Question

We have a table A - Primary Key A1, and Table B has a column B1 foreign key to A1 column.
We need to mass delete table A records which are more than 3 months old, but we do not want to delete Records from Table B, just nullify the foreign key column in table B.
So "ON DELETE CASCADE" is not applicable in our situation. Is creating a trigger with before delete option the only way to solve the above?
If yes, when inside the trigger action, how can I access the table A primary key value to find out if I have a possible match in table B? How to represent (current rows's A1 value) ?
For example - trigger action
CREATE TRIGGER DEL_A
BEFORE DELETE ON A
FOR EACH ROW
DECLARE X;
BEGIN
    SELECT COUNT(*) INTO X
    FROM B WHERE B1 = (current row's A1 value);
    IF X > 0
    THEN
       UPDATE B SET B1 = NULL
       WHERE B WHERE B1 = (current row's A1 value);
   END IF;
ENDIs the above code correct, or is there any better way to do the same?
Thanks in advance,
Rumpa

Answer 1

where B1 = :A1
You probably mean "where b1 = :old.a1" ...
Regards,
Rob.