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1.3 Relational DataBase ManageMent systeM (RDBMs)

        The concept of a relational database was developed by E.F. Codd at IBM in 1970. Relational means establishing link
        between two or more tables of a database. RDBMS is a relational DBMS in which tables are linked to each other by fields.
        In addition to all advantages of DBMS (as discussed), RDBMS helps in the management of a database in a broader way.
        Example of RDBMS is the EMPLOYEE Table


                      Name          Employee_ID          Gender            Salary         Date_of_Birth

                 Neha                   1121         Female                20000           04-03-1990

                 Himani                 3145         Female                20000           23-11-1992

                 Paras                  2134         Male                  25000           19-10-1993


        1.3.1 Key Terms of RDBMS


        Ð ÐTuple: A row is called a Tuple.
        Ð ÐAttribute: A column is called an Attribute.
        Ð ÐRelation: A table is called as a Relation.
        Ð ÐDomain: The data type of values in each column is called the Domain.

        Ð ÐDegree: The number of attributes in a relation is called the Degree of a relation.
        Ð ÐCardinality: It refers to the uniqueness of data values contained in a particular column of a database table.
        Ð ÐRelational Schema: A relational schema can be defined as a set of relational tables and associated items that are
           related to one another.

        Ð ÐRelation State: is the set of tuples in the relation at a point in time.
        1.3.2 Relational Model Constraints

        Constraints are restrictions on the values stored in a database based on the requirements. For example, in the relation
        EMPLOYEE, the Employee_ID must be a 4-digit number, the Date_of_Birth must be such that the birth year > 1985.
        There are various types of constraints in Relational model. Let us read about them.

        Ð Ð Domain Constraint: It specifies that the value of every attribute in each tuple must be from the domain of that
           attribute. For example, the Employee_ID must be a 4-digit number. Hence a value such as “12321” or “A234”
           violates the domain constraint as the former is not 4-digit long and the latter contains an alphabet.

        Ð Ð Key Constraint: Before we can explain this constraint, we need to describe the terms superkey, key, candidate key
           and primary key.

              Superkey: It is a set of attributes in a relation, for which no two tuples in a relation state have the same combination
             of values. Every relation must have at least one superkey which is the combination of all attributes in a relation.
             Thus, for the EMPLOYEE relation, following are some of the superkeys:
            (a) {Name, Employee_ID, Gender, Salary, Date_of_birth} - default superkey consisting of all attributes.
            (b) {Name, Employee_ID, Date_of_Birth}
            (c) {Employee_ID, Gender, Salary}

            (d) {Name, Employee_ID, Gender}
            (e) {Employee_ID}


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