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Each row in Table 4.1 has data about one student's preference for a game. Column headings describe the information
            stored in the columns. Thus, for each student, RollNo, Name, and Game are stored in the first, second, and third
            columns, respectively. In addition to the user data, DBMS also keeps a description of the stored data, called metadata
 Unit II: Database Query using SQL
            (also called data about data). For example, in the above example, DBMS may store the table's name, names of columns,
            and the type of data that columns contain.



                   Metadata: Description of data like names of tables, names and types of attributes.


            Database Management System (DBMS) is a collection of programs that help define, construct, and manipulate the
            database. Database definition includes specifying the structure of the database, type of data, and restrictions imposed
            on data. Database construction deals with storing the actual data in the database. Finally, database manipulation deals
            with the modification or retrieval of data. DBMS acts as an interface between end-users and the database. As every
            access to data in a DBMS must go through DBMS, it can control access to data by checking against unauthorised access
            to data. Since several end-users may access the same database, DBMS facilitates sharing  of data.

            4.2 Relational Data Model

            The  relational  data  model  founded  by  E.F.  Codd  of  IBM  is  prevalent.  It  defines  a  database  as  a  collection  of
            relations. Each relation is a table comprising of rows and columns. A relational data model comprises the following
            components:
            1.   Entity: An entity is a real-world object about which data is stored in the database. For example, a company's
               database may have employee and department  entities.
            2.   Attribute: An attribute defines the characteristics of an entity. For example, employee id, date of birth, address,
               and salary form attributes of an employee.
            3.   Relationship: A relationship specifies how two entities are linked (or related) to each other. For example, in a
               database that deals with employees of a company who work in different departments, the employee entity is
               related to the department entity since an employee works in a department.
            4.   Constraint: Constraint specifies the restrictions imposed on the data stored. For example, each employee should
               have a unique id.
            We will use the terms, relation and table interchangeably.

            4.2.1 Relational Data Model Concepts

            Let us recall the database COMPANY constructed  in class XI. We organised information about the company into four
            tables, EMPLOYEE, DEPARTMENT, PROJECT and WORKS_ON, as shown.

                                                        Relation Employee
               ID      FName     LName    Gender    Address        City     Pin_Code     DOB       Salary  Dept_No
                                                   West        Andhra
              E0001     Raj      Reddy       M                               534197   1980-06-13 100000        2
                                                   Godavari    Pradesh
                                                   Dispur,
              E0002    Dhiraj     Bora       M     Kamrup,     Guwahati      781005   1975-09-30   85000       1
                                                   Assam
                                                   8/33, Geeta
              E0003   Muskan     Taneja      F                 Delhi         110031   1990-01-25 100000        2
                                                   Colony




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