Constraints ⇒ can't record partial information

"Mark one box only" looks like an integrity constraint. But if we impose it we end up recording less information. The real world is messy; we often need relaxed constraints in order to record partial information about the world.

Functional dependencies

Consider the supplier relvar which records the extension of the following predicate:

There exists a supplier under contract identified by supplier number [SNO] with name [SNAME] having status [STATUS] and located in city [CITY]

SNO	SNAME	STATUS	CITY
S1	Smith	20	London
S2	Jones	10	Paris
S3	Blake	30	Paris
S4	Clark	20	London
S5	Adams	30	Athens

It is assumed this relvar has key {SNO}. That's a key constraint; it implies the FD constraint

    FD:   {SNO} --> {SNAME,STATUS,CITY}

Generally speaking FDs are incompatible with partial information.

Putting it another way, if we only have partial information about a given supplier, then we are unable to record any information about them using this predicate.