Implement backward-incompatible changes to a given interface.
Contains the initial files
It adds a new column to book table, called preface. It sets value from description to new filed preface because preface can't be null.
Source code deals with both description and preface variables at the same time.
It finishes migrating all description values to preface column and it renames the column description by preface.
Now, in the code, all references are set to preface. Description references are deleted in the code.
It deletes description column because it is not longer used by the application.
It adds a new field to Book table called price. This is an integer which its value can be null.
It adds a new field called extendedPrice. This is a float value that can be null, same as previous version (v5). In this case, all values in price get copied to extendedPrice.
In the code, both values are represented in class Book, while all references are set to extendedPrice.
Old implementation is deleted. There is no need to change anything in regards to the database because values for price and extendedPrice could always be null.
Field price gets deleted in database's Book table. The code does not change at this step anymore.