The relational model in DBMS is based on the concept of relations and set theory proposed by Dr. Edgar F. Codd in 1970. A relation is defined as a two-dimensional table with rows representing tuples (records) and columns representing attributes (properties). A domain defines a set of atomic values for an attribute such as data type and value range. A relation schema specifies the structure of a table with attributes. A relation instance represents the actual data in a relation at a given time which can change with inserts, deletes or updates. Other concepts include degree as the number of columns, cardinality as the number of rows, and NULL values to represent unknown data.