Keys

A key is an attribute of the relation. There are several types of keys:

• superkey

  • K is a superkey of R if values of K are sufficient to identify a unique tuple of each possible relation r(R)

  • Ex. {ID} and {ID,name} are both superkeys of instructor

• candidate key

  • A candidate key is a minimal superkey

  • Ex. {ID} is a candidate key for instructor

• primary key

  • One of the candidate keys is selected as a primary key

  • A primary key must uniquely identify a tuple

• foreign key

  • A relation R1 may include an attribute that is the primary key of another relation R2; this attribute is called the foreign key from R1 to R2

  • R1 is called the referencing relation and R2 is called the referenced relation

  • The foreign key constraint (or referential integrity constraint) requires that the values of the attribute in the referencing relation must appear in at least 1 tuple of the referenced relation

In the university database figure (see previous section), the candidate keys for each relation have been underlined and the arrows denote foreign key dependencies.