A relational database (RDB) is a database that stores its data in tables that are related by certain structural constraints. The word relational comes from the mathematical concept of a relation, which is essentially the same thing as a table. The precise definition follows.
A domain
is a set of data values of the same data type, usually integer, decimal number, or text, but could be Boolean (True/False), nominal, or URL, among others. If D1, D2 ,…, Dn are domains, then their Cartesian product
is the set D1, D2,…, Dn of all n-component sequences t = (d1, d2 ,…, dn ), where each di
∈
Di. Such sequences are called tuples (as in octuples when n = 8). A tuple is like a vector, except that the components of a tuple may be of different types; the components of a vector are usually just numbers. A relation is a subset of a Cartesian product of domains; that is, a set of tuples all from the same domain sequence (D1, D2 ,…, Dn), which is called
the schema for the relation...