I have Junctrion object called "LOCATION JUNCTION" it has two masterS like ---   Opportunity and Location when i create new junction object record like

1. name1 opp1 loca1
2. name2 opp2 loca1  then it shuld throw error like "please select different location"(FOR one account if  opp1 is having location1 then opp2 should not take location1) How to write trigger on this junnction object. Please suggest. Thanks, kiran.