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#1

Old

tlbaxter99@yahoo.com

Posts: n/a

?? Functional Dependency Question ?? - 10-18-2008 , 12:33 AM

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

Reply With Quote

#2

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#3

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#4

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#5

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#6

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#7

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#8

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#9

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

#10

Old

Walter Mitty

Posts: n/a

Re: ?? Functional Dependency Question ?? - 10-18-2008 , 08:06 AM

<tlbaxter99 (AT) yahoo (DOT) com> wrote

Quote:

Hi all,

I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
ed.". The authors discuss how, given a set if FDs, additional FDs can
be inferred. The authors provide six "Inference Rules". At one point
the authors say this:

"Although X->A and X->B implies X->AB by the union rule stated above,
X->A, and Y->B does *not* imply that XY->AB."

I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
that XY->AB.

I'm sure I'm wrong but I'm not seeing it. Can someone explain?

Thanks

If I understand it right, XY does not imply X and Y.

Reply With Quote

Reply

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