Entity integrity constraint

In the relational model, a relation is defined as a set of tuples. This means that all the tuples in a relation must be distinct. The entity integrity constraint is enforced by having a primary key, which is an attribute/set of attributes with the following characteristics:

• The attribute should be unique
• The attributes should be not null

Each relation must have only one primary key, but can have many unique keys. A candidate key is a minimal set of attributes that can identify a tuple. All unique, not null attributes can be candidate keys. The set of all attributes form a super key. In practice, we often pick up a single attribute to be a primary key instead of a compound key (a key that consists of two or more attributes that uniquely identify a tuple) to simplify the joining of the relations with each other.

If the primary key is generated by the DBMS, then it is called a surrogate key or synthetic key. Otherwise, it is called a natural key. The surrogate key candidates can be sequences and universal unique identifiers (UUIDs). A surrogate key has many advantages such as performance, requirement change tolerance, agility, and compatibility with object-relational mappers. The chief disadvantage of surrogate keys is that it makes redundant tuples possible.