Help: data validation before save or after execute query

Question

Hi,
The issue that I have are:
1. A form allows user to insert and save records. When user press the save button, Instead of have an oracle error message: "FRM-40508: ORACLE error: unable to INSERT record," I'd like there be some step taken to send an alert to user if the user does not enter the date meets the fields constraint(s).
2. Same for the execute query button, when there are no records return from the query.
Thank you.
Jimmy

Answer 1

Thank you all, 437901(error or informative?), Ajani, Denis and Steve. It worked out with ON-MESSAGE trigger by catching message_code.
Thanks again.
Jimmy
P.S.
In Oracle9i forms developer Reference Guide, the example is misleading:
ERROR_CODE Examples
** Built-in: ERROR_CODE,ERROR_TEXT,ERROR_TYPE
** Example: Reword certain FRM error messages by checking
** the Error_Code in an ON-ERROR trigger
** Trigger: On-Error
DECLARE
errnum NUMBER := ERROR_CODE;
errtxt VARCHAR2(80) := ERROR_TEXT;
errtyp VARCHAR2(3) := ERROR_TYPE;
BEGIN
IF errnum = 40301 THEN
Message('Your search criteria identified no matches...
Try Again.');
ELSIF errnum = 40350 THEN
Message('Your selection does not correspond to an employee.');
ELSE
4-170