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Unit 6: Relational Language and Database Design
Notes
Figure 6.2: E-R Diagram
ename Since dname
eno salary did budget
Employees Works -in Department
Thus, an entity (rectangles) is a “thing” or “object” in the real world that is distinguishable from
other objects, example, each employee is an entity and each department is an entity.
A relationship (diamond) is an association (connection) between different entities.
Example: Works-in a department, thus here, works-in is a relationship between two
different entities.
At last, each fields, i.e., employee, department and works-in has their own details, which are
represented by attributes (ellipses), i.e., employee details are ename (employee_name), eno
(employee_number) and salary, and, department details are dname (department_name), dno
(department_number) and budget, and, works-in details are since (the starting date) of an
employee in a department.
Task Which tool you used for your DBMS security?
6.7 Constraints
Consider the works-in relationship shown in the following figure.
Figure 6.3
ename Since dname
eno salary did budget
Employees Works -in Departmemt
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