All-relations

Reference: https://thorben-janssen.com/ultimate-guide-association-mappings-jpa-hibernate/

We can have the following relations in an rdbms:

1. one to one
2. one to many
3. many to many

And each of them can be unidirectional (from either side), or bi-directional in jpa.

Although, we can create similar relations (meaning either via FKs or Join tables) in db using bi-directional as well as uni-directional relations (from either side), unidirectional or bidirectional helps us to NAVIGATE the other entity from current entity objects within JPQL or Criteria query.

Sometimes, say if we have one to many relationship, we may not want to create a list on one-to-many side.
Imagine we have to entities - Company and Employees.
 If a company has 1000 employees, and we create a relation (uni-directional or bi-directional) from Company side, then everytime a company row is fetched, all 1000s of its employees will be loaded in RAM!
Of course LAZY and EAGER fetch type is one way to handle it, but needs to be explored.

One to many

Company, Employees
1 Company can have many employees, but 1 employee can belong to just 1 company. (legally)

#1. Unidirectional relation from Company to Employee:

  #1a. With @JoinColum - hibernate creates FK in Employee which references Comapny's id

  #1b. Without @JoinColumn - hibernate creates a join table

#2. Unidirectional relation from Employee to Company

#3. Bidirectional relation