avoid rewriting same code in trigger - sql

Question

I am writing data validations at database level using table.
Following is my table structure used for validations:(with limited records for understanding)
SRNO | COL_NAME | OBJ_NAME | IS_MANDATORY | ALTER_COL | ERROR_MSG
1 | TITLE | Customer | Y | | Enter first name!
2 | FNAME | Customer | Y | LNAME | Either first or last name are required!
3 | MNAME | Customer | N | |
4 | LNAME | Customer | Y | FNAME | Either first or last name are required!
I have written trigger which validates data before inserting it to the database as per validation table:
CREATE OR REPLACE TRIGGER VALIDATE_CUST
BEFORE INSERT ON CUSTOMER REFERENCING NEW AS NEW OLD AS OLD FOR EACH ROW
DECLARE
T_COL_VAL NUMBER(4);
BEGIN
IF INSERTING THEN
FOR C IN
(select COL_NAME,ERROR_MSG from VALIDATE_COLS where IS_MANDATORY='Y' and OBJ_NAME='Customer' order by SRNO)
LOOP
T_COL_VAL := CASE C.COL_NAME
WHEN 'TITLE' THEN nvl(length(:NEW.TITLE),0)
WHEN 'FNAME' THEN nvl(length(:NEW.FNAME),0)
WHEN 'MNAME' THEN nvl(length(:NEW.MNAME),0)
WHEN 'LNAME' THEN nvl(length(:NEW.LNAME),0)
ELSE length('OK')
END;
IF T_COL_VAL=0 THEN
RAISE_APPLICATION_ERROR(-20001,C.ERROR_MSG);
END IF;
END LOOP;
END IF;
END;
Above trigger checks whether all the fields which are mandatory have value or not. Above trigger is working fine.
Now I want to use ALTER_COL column from validations table. ALTER_COL column will be checked in case original column is null. For example either of FNAME or LNAME are mandatory. So if FNAME is not provided then trigger must check for LNAME before raising error, and only if LNAME is also blank then only raise an error.
My question is that will I need to rewrite the case statement in if condition to get the length of ALTER_COL like :
IF T_COL_VAL=0 THEN
--if `ALTER_COL` is not null
--Write case statement again to get the length of `ALTER_COL`
--If length of `ALTER_COL` is 0 then only raise following error
RAISE_APPLICATION_ERROR(-20001,C.ERROR_MSG);
END IF;
Or is there any better way of doing this?
Any suggestion will be appreciated!
Update1(after the answer from #Wernfried)
I tried writing function which can avoid rewriting code but I can't pass NEW to the function. Function which I tried looks like.
CREATE FUNCTION FN_VALIDATE_CUST(COL_NAME in VARCHAR) return NUMBER as
BEGIN
return CASE COL_NAME
WHEN 'FNAME' THEN nvl(length(:NEW.FNAME),0)
WHEN 'MNAME' THEN nvl(length(:NEW.MNAME),0)
WHEN 'LNAME' THEN nvl(length(:NEW.LNAME),0)
WHEN 'TITLE' THEN nvl(length(:NEW.TITLE),0)
ELSE length('OK')
END;
END;
I can't pass the NEW object to the above function to access column value. So I will need to write case statement in trigger event if I use function.

Answer 1

Or is there any better way of doing this?
Definitely -- use NOT NULL and CHECK constraints in the database to do this. That's what they're for, and they're a lot more performant, reliable, and flexible than your approach.
If you really need to provide custom error messages for constraint violations then ensure that the constraints are named, and provide a table of custom messages to be used when the constraint is violated.
Edit ...
If enforcement of the constraints is dependent on the presence of a customer number in another table, then I'd rather have a flag in the table that indicates whether or not to apply the check and not null constraints, and include that in the constraint definitions.
For example:
... add constraint lname_required check (apply_constraints = 0 or lname is not null)
I don't think that in this case you can get away from needing a trigger, as you'd have to maintain the value of "apply_constraints" with one, based on the presence of the customer number in the other table (to ensure that deletions or insertions on that table update the relevant values in this table).

Answer 2

You can write a procedure and call this procedure in your trigger.