Ãîëîâíà |
Consider R = {A, B, C, D, E} with a set of FDs F = {AB-> DE, C-> E, D-> C, E-> A}
And we wish to project those FDs onto relation S = {A, B, C}
Give the FDs that hold in S
Hint:
We need to compute the closure of all the subsets of {A, B, C}, except the empty set and ABC.
Then, we ignore the FD's that are trivial and those that have D or E on the RHS
гøåííÿ
Calculating F + for a Sub-Relations
R = {A, B, C, D, E}
F = {AB-> DE, C-> E, D-> C, E-> A}
S = {A, B, C}
A + = A
B + = B
C + = CEA [C-> E, E-> A]
AB + = ABDEC [AB-> DE, D-> C]
AC + = ACE [C-> E]
BC + = BCEAD [C-> E, E-> A, AB-> DE]
We ignore D and E.
So, the FDs that hold in S are:
{C-> A, AB-> C, BC-> A}
(Note: BC-> A can be ignored because it follows logically from C-> A)
Ðîçãëÿíüòå R = {A, B, C, D, E} ç ðÿäîì FDs F = {AB-> DE, C-> E, D-> C, E-> A}
² ìè õî÷åìî ñïðîåêòóâàòè ò³ FDs íà ñòàâëåííÿ S = {A, B, C}
Äàéòå FDs, ÿê³ òðèìàþòüñÿ â S
íàòÿê:
Ìè ïîâèíí³ îá÷èñëèòè çàêðèòòÿ âñ³õ ï³äìíîæèí {A, B, C}, êð³ì ïîðîæíüîãî íàáîðó ³ ABC.
Ïîò³ì ìè ³ãíîðóºìî FD's, ÿê³ òðèâ³àëüí³ ³ ò³, ó ÿêèõ º D àáî E íà RHS
гøåííÿ
Îá÷èñëåííÿ F + äëÿ ïîäîòíîøåí³ÿ
R = {A, B, C, D, E}
F = {AB-> DE, C-> E, D-> C, E-> A}
S = {A, B, C}
A + = A
B + = B
C + = CEA [C-> E, E-> A]
AB + = ABDEC [AB-> DE, D-> C]
AC + = ACE [C-> E]
BC + = BCEAD [C-> E, E-> A, AB-> DE]
Ìè ³ãíîðóºìî D ³ E.
Òàê, FDs, ÿê³ òðèìàþòüñÿ â S:
{C-> A, AB-> C, BC-> A}
(Ïðèì³òêà: BC-> A ìîæå áóòè ïðî³ãíîðîâàíèé, òîìó ùî â³í ñë³ä ëîã³÷íî â³ä C-> A)
ÂÀвÀÍÒ 23 (ÐÊ 1 / Ñåìåñòð 1)
Çíàéä³òü íåïðèâîäèìîãî ïîêðèòòÿ áåçë³÷³ ôóíêö³îíàëüíèõ çàëåæíîñòåé S = {AB-> D, B-> C, AE-> B, A-> D, D-> EF}, çàäàíèõ äëÿ çì³ííî¿-â³äíîñèíè R (A, B, C, D, E, F).
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