Truncate field before insert

Question

Suppose I have the following:
table: mytable
field: username (VARCHAR2(35))
field: zip (VARCHAR2(5))
I am importing data from an excel file with columns username and zip. A couple of the usernames contain more than 35 characters, and some of the zipcodes are in the xxxxx-xxxx format, thus containing more than 5 characters.
I attempted to write a before insert trigger to truncate the data before they are inserted. For any username that is above 35 characters from the excel file, just truncate the username to the first 35 characters. Similarly, for any zip code that is above 5 characters, just truncate the zip to the first 5 characters.
The trigger for the zip codes:
CREATE OR REPLACE
TRIGGER trigger_zipc
  BEFORE INSERT
  ON mytable
  FOR EACH ROW
BEGIN
  SELECT substr(:new.zip, 1, 5)
  INTO :new.zip
  from dual;
END;
However, I get the error: ORA-12899: value too large for column
I've read on other forums that this is what happens, and that a possible work around is to use INSERT INSTEAD??
Can anyone give me a solution or point me in the right direction?

Answer 1

Can anyone give me a solution or point me in the right direction?
Sure - the 'right direction' is to mark this thread ANSWERED and repost your question in the Sql and Pl/Sql forum:
PL/SQL
This forum is ONLY for Sql Developer questions and your question doesn't appear to have anything to do with that tool.
As the exception says the value is too large for the column. Your code that tries to trim the value will NEVER be executed because the exception occurs BEFORE it ever gets that far.
The buffer that Oracle creates to hold that :NEW value is too small to hold the value you try to insert - that is why you get the exception.
You can NOT use a trigger to perform validations such as those since the data will NEVER make it into the buffer the trigger uses.before it ever executes your code.