How do you effectively model inheritance in a database?

Question

What are the best practices for modeling inheritance in databases?

What are the trade-offs (e.g. queriability)?

(I\'m most interested in SQL Server and .NET,

Answer 1

You would normalize of your database and that would actually mirror your inheritance. It might have performance degradance, but that's how it is with normalizing. You probably will have to use good common sense to find the balance.

Answer 2

repeat of similar thread answer

in O-R mapping, inheritance maps to a parent table where the parent and child tables use the same identifier

for example

create table Object (
    Id int NOT NULL --primary key, auto-increment
    Name varchar(32)
)
create table SubObject (
    Id int NOT NULL  --primary key and also foreign key to Object
    Description varchar(32)
)

SubObject has a foreign-key relationship to Object. when you create a SubObject row, you must first create an Object row and use the Id in both rows

EDIT: if you're looking to model behavior also, you would need a Type table that listed the inheritance relationships between tables, and specified the assembly and class name that implemented each table's behavior

seems like overkill, but that all depends on what you want to use it for!

Answer 3

Note that some database engines already provides inheritance mechanisms natively like Postgres. Look at the documentation.

For an example, you would query the Person/Employee system described in a response above like this:

  /* This shows the first name of all persons or employees */
  SELECT firstname FROM Person ; 

  /* This shows the start date of all employees only */
  SELECT startdate FROM Employee ;

In that is your database's choice, you don't need to be particularly smart !