trigger check date in stock

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Oracle

hi, guy and anybody help me to create a trigger.

example I have data:

ITEM.......QTY.......DATE
ABC........100......1-jan-2012
ABC........150......15-jan-2012

on 25 jan 2012, ABC sold out 120, so i want data above remain like this:

ITEM.......QTY.......DATE
ABC........0.........1-Jan-2012
ABC........30........15-Jan-2012

I'm new for oracle, please help me guys,

Thanks in advance.
Sorry for my bad English.