It can be a single-user database system, which supports one user at a time, or a multiuser
database system,

was introduced by C. F. Codd in 1970. Currently, it is the most widely used data model

The order of attributes has significance

Each attribute has … name;

Each tuple is distinct; there are no duplicate tuples;

The order of tuples has significance, theoretically

Each cell of the relation contains exactly one atomic (single) value;

The values of an attribute are all from the same domain;

The candidate key that is selected to identify tuples uniquely within the relation.

Candidate key: when a key consists of more than one attribute

Super key : An attribute, or set of attributes, that uniquely identifies a tuple within
a relation.

A super key such that no proper subset is a super key within the relation.

in each tuple of R, the values of K uniquely identify that tuple

Additional rules specified by the users or database administrators of a database that define or constrain some aspect of the enterprise.

View : The dynamic result of one or more relational operations operating on the base relations to produce another relation

Represents a value for an attribute that is currently unknown or is not applicable for this tuple.

Null Can create problems when functions such as COUNT, AVERAGE and SUM are used

Null can represent