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Last updated on 5/5/22

Transform Your Inheritance Relations

The relational model wasn't designed to show inheritance relationships. It’s possible to model them, but it requires you to do some thinking.

There are three different ways to translate an inheritance relationship:

  • By reference.

  • By a parent class.

  • By child classes.

Each of these three methods has pros and cons, which we’ll go through now.

The most common method is to transform inheritance by reference because this suits most scenarios.

Transform Your Inheritance by Reference

When using this method, you need to create a table for the parent class and a one for the child class. Foreign keys in the child class tables will allow you to refer back to the parent class table.

The primary key of each child table must be the same primary key as the parent table. Additionally, each primary key in the child tables is also a foreign key referencing the parent table.

What does this mean?

In our case, the primary key of  production  is (  title  ,  production_company  ). Therefore, you need to add these two attributes to the child tables (tv_film,  feature_film  ,  web_series  ,    tv_series  , and  series  ), and they will also be both the foreign key to  production  and the primary key to each child table:

Inheritance by reference
Inheritance by reference

Also, consider that  season  is part of the  series  primary key, but we don’t want to make this course too long, so we won’t deal with that here.

Transform Your Inheritance by Parent Class

There's just one table that corresponds to the parent class when using this method.

This requires bringing the data from the child classes into a single table and incorporating the attributes of the child classes into the parent class. You also add an additional attribute (the discriminator attribute), indicating which child class each row corresponds to.

Looking back at your UML, you’ll see that all the  production  child classes have only one attribute:  season  . So, you’ll add this to the  production  table. Note that this attribute will have a  NULL  value when the production isn’t a series.

So, you add a discriminator attribute that tells you if a production is a TV film, a feature film, etc. Call this attribute  filming_type  :

The production table with the discriminator attribute filming_type
The production table with the discriminator attribute filming_type

Transform Your Inheritance by Child Class

When using this method, the parent class doesn't generate a table. Instead, each child class is translated into a table, and each of these tables inherits the attributes of the parent class.

What’s more, the primary key of each child table must match the primary key of the parent class. So in the scenario, each child table will have  (title, production_company)  as a primary key:

Inheritance by child class: the parent production and series classes don’t become tables
Inheritance by child class: the parent production and series classes don’t become tables

Alternatives

The relational model wasn’t designed to manage inheritance, so the object-relational model was created. This model builds upon the relational model and adds ideas borrowed from the object-oriented approach (as modeled by the UML diagram), such as inheritance. 

Now it's Your Turn!

Now that you have the rules for translating from UML to the relational model, it is your turn to can translate your UML diagram into a relational model!

You can then compare your relational model with the answer below:

Answer
Answer

Let’s Recap!

An inheritance relationship can be translated in three different ways:

  • By reference.

  • By parent class.

  • By child classes.

Well done! You now know how to translate any UML diagram into a relational model. But actually, does your model minimize redundancy? That’s what we’re going to explore in the final part.

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