In a relational database each table must have an attribute (or a set of) uniquely identifying each row.
There are several related terms:
- superkey: Any set of columns which has/will-have different values in each row.
- candidate key: A minimal superkey; such a set of columns that when a column is removed then there are/will-be duplicate rows. A table might have several candidate keys.
- primary key: A chosen candidate key. The column names of a table primary key are often shown underlined.
A key is composite if it contains more than one column.
For a particular table and its candidate keys:
- prime attribute: a column belonging to any candidate key
- non-prime attribute: does not belong to any candidate key
Observation: a DBMS can generate a column (often named id) with values guaranteed to be unique.