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Why Object Relational Mapping (ORM)?

Why Object Relational Mapping (ORM)?

When we work with object-oriented systems, there's a mismatch between the object model and the relational database. RDBMSs represent data in a tabular format whereas object-oriented languages, such as Java or C# represent it as an interconnected graph of objects. Consider the following Java Class with proper constructors and associated public function:

public class Employee {
   private int id;
   private String first_name;
   private String last_name;  
   private int salary; 

   public Employee() {}
   public Employee(String fname, String lname, int salary) {
      this.first_name = fname;
      this.last_name = lname;
      this.salary = salary;
   public int getId() {
      return id;
   public String getFirstName() {
      return first_name;
   public String getLastName() {
      return last_name;
   public int getSalary() {
      return salary;

Consider above objects need to be stored and retrieved into the following RDBMS table:

create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,

First problem, what if we need to modify the design of our database after having developed few pages or our application? Second, Loading and storing objects in a relational database exposes us to the following five mismatch problems.

Sometimes you will have an object model which has more classes than the number of corresponding tables in the database.
RDBMSs do not define anything similar to Inheritance which is a natural paradigm in object-oriented programming languages.
A RDBMS defines exactly one notion of 'sameness': the primary key. Java, however, defines both object identity (a==b) and object equality (a.equals(b)).
Object-oriented languages represent associations using object references where as am RDBMS represents an association as a foreign key column.
The ways you access objects in Java and in a RDBMS are fundamentally different.

The Object-Relational Mapping (ORM) is the solution to handle all the above impedance mismatches.