Question

Which design would you choose for a database for the custom fee table and why?

Option 1) 1 table, UserId determines if user has custom fee

ACCOUNT_FEE
TypeId
FixedRate
VariableRate
CurrencyId
UserId (UserId is null for default account fee, UserId is present for custom fee)

Option 2) 2 separate tables, one for default fee the other for user’s custom fee

ACCOUNT_DEFAULT_FEE
TypeId
FixedRate
VariableRate
CurrencyId

USER_ACCOUNT_FEE
TypeId
FixedRate
VariableRate
CurrencyId
UserId

Answer 1

I would do it differently. Create a user called “Default Account” and give it a specific ID, or even “0” (identity_insert may be required). This allows the table to follow referential integrity rules AND the index on UserID can quickly pick out the Default Account Fee.

Otherwise,

• using a single table, it will have to table-scan just to get WHERE UserID is null
• using two tables, you have to keep them separate yet in sync when they really just logically store the same data