Answer:
1 .The main purpose of primary key is to identify the uniqueness of a row, where as unique key is to prevent the duplicates, following are the main difference between primary key and unique key.
Step-by-step explanation:
2. Field size means the dimensions along the major axes of an area in a plane perpendicular to the central axis of the useful beam of incident radiation at the normal treatment distance and defined by the intersection of the major axes and the 50 percent isodose line.