Question

For example I have 2 tables, Users and UserRelations, and it is a one to many relationship.

For the UserRelations table, I can have an identity column and make it the primary key:

[RelationID] [int] IDENTITY(1,1) NOT NULL, [UserID] [int] NOT NULL, [TargetID] [int] NOT NULL, 

Or I can design the table like:

[UserID] [int] NOT NULL, [TargetID] [int] NOT NULL, 

and make UserID + TargetID the primary key.

My question is what are the implications of going with each design, which is better for performance?

Answer 1

If you use the former design, with the superfluous identity column, there’s no constraint against inserting two rows with identical UserID and TargetID. You’d have to create a UNIQUE constraint over the other two columns, which creates a compound index anyway.

On the other hand, some frameworks (e.g. Rails) insist that every table has a surrogate key named id so the ‘correct’ design may not work. It depends on what code you’re writing to use this table design.