Relational Model ( Database system concepts )

Example of a Relation

Attribute Types

* Each attribute of a relation has a name

* The set of allowed values for each attribute is called the domain of the attribute

* Attribute values are (normally) required to be atomic; that is, indivisible

lE.g. the value of an attribute can be an account number,
but cannot be a set of account numbers

* Domain is said to be atomic if all its members are atomic

* The special value null  is a member of every domain

* The null value causes complications in the definition of many operations

** We shall ignore the effect of null values in our main presentation and consider their effect later

Relation Schema

* Formally, given domains D1, D2, …. Dn a relation r is a subset of
        D1 x  D2  x … x Dn
Thus, a relation is a set of n-tuples (a1, a2, …, an) where each ai  Î Di

* Schema of a relation consists of

# attribute definitions

4name

4type/domain

# integrity constraints

Relation Instance

* The current values (relation instance) of a relation are specified by a table

* An element t of r is a tuple, represented by a row in a table

* Order of tuples is irrelevant (tuples may be stored in an arbitrary order)