Object-oriented programming and database management

ORM(Object Relational Mapping) in Hibernate is a good example for your clarification!

For the questions like : Is a class diagram essentially a database design? and Are classes the equivalent to tables and their attributes equivalent to columns?.. Yes Let me explain with you an example:

class Department
{
     private int departmentNo;
     private int departmentName;
     private Set<Student> students;
}

class Student
{
    private int studentId;
    private int studentName;
    private Department department;
}

using these entities, Two database tables will be generated with hibernate. by default,

Department: departmentno(int),departmentname(varchar(255))
Student: studentid(int), studentname(varchar(255)),departmentno(int) [foreign key]

are generated.

List of Students of a particular department can be retrived by,

Department department = ......; // hibernate related thing which hit the database
Set<Student> = department.getStudents();

I have been reading on this for some years now. I love the OOP approach and I also develop database-driven applications. Here are my findings:

1. OOP and Data Model in a sense are two separate worlds. This is because RDBMS have existed for years before OOP was adopted.

2. OOP focuses on classes and objects; RDBMS focuses on data storage and retrieval.

3. If you spend time building classes that model your tables, its fine,but that would add a performance overhead to your application and increase the development time.

My Advice for .Net Developer: Use that classes that have already been provided by the .Net framework under the System.Data namespace. A good knowledge of these classes would reduce the size of your code to the minimum. Only create a few custom classes when strictly necessary