Object model and Relational models (RDBMS) have following

problems that are part of Object Relational Impedance Mismatch:

Granularity: Object model is more granular than Relational model.

There are more classes in object model than the corresponding

tables in relational model.

Inheritance: Object model supports inheritance. But Relational

model does not have any concept of inheritance.

Identity: Relational model has just one criteria for sameness of data.

It is based on primary key. In object model like Java we can have

equals as well as == for sameness of objects.

Associations: In Object model associations are uni-directional. In

RDBMS, there is a concept of foreign key for association. Also

multiplicity of a relationship is hard to judge by looking at object

model.

Data navigation: In Object model, you can move from one object to

another object for getting data. Egg. you can retrieve and Employee

object, then go to its department object and then get the employees

in the department object. In RDBMS, we try to minimize the SQL

calls, so we get all the data by using joins

https://medium.com/booleanbhushan/object-relational-impedance-mismatch-28fc2440dee4