Types of mappings

1. One-to-one Mapping:
   This is the simplest mapping between two objects.
   
   For example: every person has a unique finger-print.
   So if person and finger-prints are two separate tables in a database, then it can be said that those two tables have a one-to-one mapping.
   Typically, one-to-one mapping fields are recommended to be put in the same table to avoid costly joins when reading.
   
   
   
2. One-to-many Mapping:
   Consider an author who has written several books.
   If authors and books are kept in two tables, then there is a one to many mapping from author table to book table.
   Such a relationship is stored using:
   
   1. A third table storing author-id vs book-id.
   2. Storing a foreign key, such as author-id in book table.
   
   
   
3. Many-to-one Mapping:
   This is just the inverse of the one-to-many mapping.
   For instance, in the above case, book table has many-to-one mapping with the author table.
   
   
   
4. Many-to-many Mapping:
   Consider a relationship between a person and the types of car owned by him.
   Clearly, it is a one-to-many relationship because a single person can own different types of cars like sedan, coupe etc.
   
   But the relationship is one-to-many from car-type table to persons table also because a given car-type can be owned by many individuals.
   Example, sedan can be owned by more than one persons.
   
   Thus, in such a case, there exists a many-to-many relationship between person and car-type table.