Question

I have one table called Player

• PlayerID (PK)
• More columns

I want to create another table that has a 1 to 1 relationship to Player named PlayerExtraInfo to contain some columns. So I have several options:

1. Instead of creating a new table, add PlayerExtraInfo columns to Player table

2. Create PlayerExtraInfo table and have a FK in Player table.

   Player table

   • PlayerID (PK)
   • More columns
   • PlayerExtraInfoID (FK)

   PlayerExtraInfo table

   • PlayerExtraInfo (PK)
   • More columns

3. The opposite: PlayerExtraInfo contains a FK to Player table. To get sure that relationship remains 1 to 1 I add a unique constrain.

   Player table

   • PlayerID (PK)
   • More columns

   PlayerExtraInfo table

   • PlayerExtraInfo (PK)
   • More columns
   • PlayerID (FK, Unique)

4. Similar to option 3 but mixing PK and FK in one. In this case my foreign key becomes also the primary key:

   Player table

   • PlayerID (PK)
   • More columns

   PlayerExtraInfo table

   • PlayerID (PK, FK)
   • More columns

I know that options 1, 2 are correct, but due to performance problems I am option for option 3 or 4. And thinking about these 3 and 4 options some doubts about option 4 assault me:
So my questions are:

• Are options 3 and 4 correct?
• Does option 4 break any normal form?
• Is there another option I didn’t think about?

Answer 1

When extending one table with another one it is absolutely fine (and recommended as well) to use the primary key as the foreign key.