Such "tables" would more typically be constructed as views (materialised if performance requires it) over underlying data. In this case the underlying data would have a row for each sale, with a normal nullable column for agent_id. The query that constructs the view would use GROUP BY, and since NULLs group together in GROUP BY it would work as you expect (grouping "no agent" sales together into a single record in the view). The database system thus handles ensuring data consistency, merely by using its features as they are designed to be used.