Question

suppose i am modeling a database and i have an entity Equipement, it can be a Workstation or a Peripheral, when translating to SQL, what is the best way ? Equipement table with the common attributes and the two others ? or just the two child tables?

Answer 1

You can use either a universal table, vertical partitioning or horizontal partitioning. A universal table has ALL attributes and an additional type attribute. This attribute represents which type your entity has. Attributes which you do not have for a special type are NULL. In your example you could have a table:

Equipment(e_id, attr_general,attr_workstation,attr_peripheral,type)

With following Rows:

e_id | attr_general | attr_workstation | attr_peripheral | type
---------------------------------------------------------------
  1  |    valueG    |      valueW      |       NULL      |  'W'
  2  |    valueG    |      NULL        |       valueP    |  'P' 

If you use vertical partitioning you map all your classes to tables and reference them with you base entity:

Equipment(e_id,attr_general) -> PK is e_id

Workstation(w_id,attr_workstation) -> PK,FK w_id where FK referencing to e_id

Peripheral(p_id,attr_peripheral) -> PK,FK p_id where FK referencing to e_id

If you use horizontal partioning you reference again to your base entity but you take all
attributes from each base entity as well:

Equipment(e_id,attr_general) -> PK is e_id

Workstation(w_id,attr_general, attr_workstation) -> PK,FK w_id where FK referencing to e_id

Peripheral(p_id,attr_general,attr_peripheral) -> PK,FK p_id where FK referencing to e_id

I personally use universal tables for entities which does not have many different attributes. If it comes that you have too many different attributes I would avoid this type, because you will have a lot of NULL fields.

Hope that could help you!