i need to generate a sequence for a field; i.e: i have a field 'class' , where i saved data class1, class2,class3... what i need is that , each created class will have its own sequence so that students of class1 will have ids 001, 002 ,003,.. and that of class2 will have ids 001, 002 ,003,.. please any help to generate a sequence for each class??