The relations contain attributes. Each attribute has a domain. The mathematical definition of a relation is the cartesian product of all the values in a domain for each attribute. The database is the subset of the Cartesian product that represents the entries in the database. A record or row is called a tuple.
Null value is in the domain of all attributes.
All the key definitions from the ER model carry over into the relational model. There exist both procedural and non-procedural query languages for the relational model. The languages map very closely from one to the other. For our purposes, we will focus on a procedural language, the Relational Algebra. The Tuple Relational Calculus and The Domain Relational Calculus are also presented in the text.