The values of generated surrogate keys have no relationship to the real-world "meaning" of the data held in a row. When inspecting a row holding a foreign key reference to another table using a surrogate key, the meaning of the surrogate key's row cannot be discerned from the key itself. Every foreign key must be joined to see the related data item. If appropriate database constraints have not been set, or data imported from a legacy system where referential integrity was not employed, it is possible to have a foreign-key value that does not correspond to a primary-key value and is therefore invalid. (In this regard, C.J. Date regards the meaninglessness of surrogate keys as an advantage.)