Relational Model
A relational model, also known as a relational schema, is defined by the relations between the attributes between different entities, all of which are defined in the ER Diagrams. A relation schema, as you can guess by the name are defined using relations described with the form $R(A_1,…,A_n)$in which $R$ is a relation and $A_n$ are attributes. Since relational schemas are basically a collection of different relations, they are represented as $S(R_1, R_2, R_3 …, R_n)$ where $R_n$ are relations. The actual instance of a database is represented by tuples made from the attributes’ domains. A relational model also has Integrity Constraits represented by $C$ which must be satisfied by all instances of S.