Strong entity set

If an entity type has a key attribute specified then it is a strong entity type. For example, at a university or college we have student entities and we would define a student entity type. Universities and Colleges assign unique student numbers, one per student. Student has a key attribute and would be considered a strong entity type.

Primary key

• The primary key of a relational table uniquely identifies each record in the table.
• It can either be a normal attribute that is guaranteed to be unique (such as Social Security Number in a table with no more than one record per person) or it can be generated by the DBMS.
• Primary keys may consist of a single attribute or multiple attributes in combination.