Re: Basic question?What 's the key if there 's no FD(Functional Dependencies)?

A second...

Let R1 relation of RealPeople, R2 relation of Impostors, R3 relation of Impostors and RealPeople expressed as R3 = R1 UNION R2 R1 and R2 are disjoint therefore:
R1 INTERSECT R2 = Empty Set
R1 MINUS R2 = R1
R2 MINUS R1 = R2 implies...

R1 MINUS R2
UNION
R2 MINUS R1
UNION
R1 INTERSECT R2 is the same as

R1 UNION R2 therefore R3 (call it AllPeople). In english ? Can relation AllPeople defined as the union of Impostors AND RealPeople be updated? Answer YES. Is it updated as an Impostor NO Is it updated as RealPeople NO. It is simply updated as AllPeople. Period.

Neither R1 nor R2 are updated in fact. A major advantage of RM is the ability to work in closure with sets (meaning independently from values included in the sets at the first place)

For instance,

R1:{A, B, C}
R2:{D, E, F}

R3 = R1 UNION R2 --> leads to R3 :{A, B, C, D, E, F} If inserting R4: {G}
Therefore {A, B, C, D, E, F} UNION {G} you produce R5 ={A, B, C, D, E, F, G}
Neither R1, R2, R4 are updated in the process just R3 and R5 are exrpressed

A consequence of assimiliating relations to revalues sets instead of focusing on their transformational properties is a common error in RM theory. Operations between 2 relations necessarily produce a new relation. Even an update to such relation produces a new relation. Received on Wed Nov 08 2006 - 22:47:53 CET