Doubt on record level validation, pl. help

Question

Hi,
I have base table block with 5 records displayed with a scroll bar but number of records not fixed depends up on requiremnet.. looks like,
Main A/C Cd Sub A/C Cd Amount
I want to have a validation which should not allow users to enter same combination of Main A/C Cd and Sub A/C Cd,
For Ex:
Main A/C Cd Sub A/C Cd Amount
100181 CM0003 200.00
100181 HK0001 100.00
100181 HK0001 200.00
In this example second and third records are having same
Main A/C Cd and Sub A/C Cd which shd allow. As soon as they enter Main A/C Cd, Sub A/C Cd and enter or move to Amount field alert should be displayed.
Hope it is a very clear example
please help as soon as possible.
Thanks in advance

Answer 1

Thanks to every one for the help.
let me make it clear, I want validation at form level only i.e. while entering records. Normally users enter at a time more than one record and finally after entering some transactions they save to database.
So what I want is as soon as they enter a duplicate of combination of MAIN A/C CD and SUB A/C CD, when they leave SUB A/C CD field there should be a alert saying that you entered a duplicate record and raise form trigger failure.
So in this way user immediately correct his mistake rather after entering all aound 20 records.
basically it has to store MAIN A/C CD and SUB A/C CD either cancating both into a single variable for each record and after entering second record it should check with first record, is that enetered values are same, if same then raise alert else next record.
Hope it is clear. but i am not able store into variable and check. I never used PL/SQL table or any arrays. please give me some idea on this.
once again thanks for spending time...