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devops  architecture  software-development  best  details 
6 weeks ago by hellsten
.net - How do you effectively model inheritance in a database? - Stack Overflow
Proper database design is nothing like proper object design.

If you are planning to use the database for anything other than simply serializing your objects (such as reports, querying, multi-application use, business intelligence, etc.) then I do not recommend any kind of a simple mapping from objects to tables.

Many people think of a row in a database table as an entity (I spent many years thinking in those terms), but a row is not an entity. It is a proposition. A database relation (i.e., table) represents some statement of fact about the world. The presence of the row indicates the fact is true (and conversely, it's absence indicates the fact is false).

With this understanding, you can see that a single type in an object-oriented program may be stored across a dozen different relations. And a variety of types (united by inheritance, association, aggregation, or completely unaffiliated) may be partially stored in a single relation.

It is best to ask yourself, what facts do you want to store, what questions are you going to want answers to, what reports do you want to generate.

Once the proper DB design is created, then it is a simple matter to create queries/views that allow you to serialize your objects to those relations.

Example:

In a hotel booking system, you may need to store the fact that Jane Doe has a reservation for a room at the Seaview Inn for April 10-12. Is that an attribute of the customer entity? Is it an attribute of the hotel entity? Is it a reservation entity with properties that include customer and hotel? It could be any or all of those things in an object oriented system. In a database, it is none of those things. It is simply a bare fact.

To see the difference, consider the following two queries. (1) How many hotel reservations does Jane Doe have for next year? (2) How many rooms are booked for April 10 at the Seaview Inn?

In an object-oriented system, query (1) is an attribute of the customer entity, and query (2) is an attribute of the hotel entity. Those are the objects that the would expose those properties in their APIs. (Though, obviously the internal mechanisms by which those values are obtained may involve references to other objects.)

In a relational database system, both queries would examine the reservation relation to get their numbers, and conceptually there is no need to bother with any other "entity".

Thus, it is by attempting to store facts about the world—rather than attempting to store entities with attributes—that a proper relational database is constructed. And once it is property designed, then useful queries that were undreamt of during the design phase can be easily constructed, since all the facts needed to fulfill those queries are in their proper places.
database-design  architecture  inheritance  software-development  details  best 
june 2015 by hellsten
The Architecture of Open Source Applications
Our goal is to change that. In these two books, the authors of four dozen open source applications explain how their software is structured, and why. What are each program's major components? How do they interact? And what did their builders learn during their development? In answering these questions, the contributors to these books provide unique insights into how they think.
architecture  software  book  open-source  software-development 
july 2014 by hellsten

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