When we are working with an object –oriented systems, chances are that there may be a mismatch of object model and the relational database.

Since RDBMS represents data in a tabular format whereas object oriented languages like Java, C++, C# represents it as an interconnected graph of objects. 

For instance consider the case of Person class here, it consist of private properties, constructors and public methods.

public class Person {
      private int id;
      private String name;
      private String address;
      private int age;
      public Person(int id, String name, String address,int age){
            this.id = id;
            this.name = name;
            this.address = address;
            this.age = age;
      }
      public int getId() {
            return id;
      }

      public String getName() {
            return name;

      }

      public String getAddress() {
            return address;

      }

      public int getAge() {
            return age;

      }


} 

Now this above object need to be stored and retrieved into a RDBMS table

create table PERSON (

   id INT NOT NULL auto_increment,
   name VARCHAR(20) default NULL,
   address  VARCHAR(20) default NULL,
   age     INT  default NULL,
   PRIMARY KEY (id)

);

1. If we analyze this model carefully, there is one problem. What if we need to modify the design of our database after having developed few pages or our application?

2. Also, loading and storing objects in a relational database exposes us to five other mismatch problems:

a. Inheritance: RDBMSs don’t define anything similar to inheritance which is a natural paradigm in OOPS languages.

b. Granularity: Sometimes we will have an object model which has more classes than the number of corresponding tables in the database.

c. Navigation: The ways we access objects in java and in a RDBMS are fundamentally different.

d. Identity: A RDBMS defines exactly one notion of ‘sameness’, the primary key. But java, defines both object identity (x == y) and object equality (equals(y)).

e. Associations: Object-oriented languages represent associations using object references where as a RDBMS represent an association as a foreign key.