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

There are two main types of inheritance you can setup in a DB, table per entity and table per Hierarchy.

Table per entity is where you have a base entity table that has shared properties of all child classes. You then have per child class another table each with only properties applicable to that class. They are linked 1:1 by their PK's

Table per hierarchy is where all classes shared a table, and optional properties are nullable. Their is also a discriminator field which is a number that denotes the type that the record currently holds

SessionTypeID is discriminator

Target per hierarchy is faster to query for as you do not need joins(only the discriminator value), whereas target per entity you need to do complex joins in order to detect what type something is as well as retreiuve all its data..

Edit: The images I show here are screen shots of a project I am working on. The Asset image is not complete, hence the emptyness of it, but it was mainly to show how its setup, not what to put inside your tables. That is up to you ;). The session table holds Virtual collaboration session information, and can be of several types of sessions depending on what type of collaboration is involved.