Primary Keys

• A Primary Key is an attribute that uniquely identifies a single object within the entity collection.
• Example: a student's student ID is unique as no other student will have the same ID number. 
• identifier key
• each entity should have at least one primary key.
• Primary Keys are drawn as normal attributes but with the name of the attribute underlined.
• Primary Keys cannot be null, a null value is an empty value (0 and spaces are values and is NOT the same as a null value.)
• Each entity must have at least one Primary Key. 
• If there is no obvious attribute to use as a primary key, we can usually just create some sort of ID number.