An entity can have many attributes, but only some attributes uniquely identify occurrences of that entity. There might be more than one unique key in an entity. For example, the EMPLOYEE entity has two unique keys, EMP ID and EMP SS NUM.
For each entity, choose from among its attributes the ones that uniquely identify each occurrence. The attribute that best serves this purpose is a good candidate for a primary key. If there is no attribute that uniquely identifies an entity, it might be necessary to combine two or more attributes for a unique key or create an attribute that serves as a key.
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