Chng 6 M hnh Thc th & Lin ktPhn 1: Gii thiu

Mc chNm c: ngha ca m hnh thc th lin kt (MH TTLK)Cu to ca MH TTLKCch xy dng MH TTLKVn dng MH TTLK vo giai on m hnh ha d liu*

Cc ni dung chnhMc ch ca m hnh thc th lin ktCu to ca MH TTLK

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1. Mc ch ca m hnh thc th lin ktN c dng trong giai on Phn tch h thng, ngay sau giai on Thu thp cc yu cu ca h thng.N c dng phn tch yu cu v d liu, nhm lm r loi y/c ny ca ngi s dng.N c dng lm cng c h tr vic trao i, thng nht cc y/c v d liu gia nh pht trin v ngi dng.N ng vai tr trung gian gia cc y/c d liu tru tng giai on kho st v cc chi tit d liu giai on thit k.*

1. Mc ch ca m hnh thc th lin ktThit k d liuPhn tch d liuThu thp cc yu cuKho st h thngPhn tch h thngThit k h thngM hnh thc th lin ktM hnh quan hCc Kch bn s dngT in d liu*

1. Mc ch ca m hnh thc th lin ktC th hn, m hnh ny s gip:Xc nh r cc n v thng tin nghip v ca h thngXc nh r cc mi quan h, rng buc gia cc n v thng tin nyTrnh by r rng v c ng cc thng tin trnT gip pht hin sm, v trnh c cc sai st, hiu lm v cc y/c h thng*

2. Cu to ca M hnh Thc th Lin kt Gm c 3 thnh phn chnh:

Thc th (Entities)Lin kt (Relationships)Thuc tnh (Attributes)*

Thc th v tp thc thMt thc th l:iu g m quan trng i vi nghip v, nn d liu v n cn phi c bit.Tn ca cc th m bn c th lit k.C th l mt i tng c th hoc tru tng.Thng l danh t.Mt tp thc th (Entity Set): l mt tp hp cc thc th cng mt loi. Tp thc th cn c gi l Kiu thc th (entity type).K hiu: thc th v kiu thc th u c biu din bng mt hnh ch nht. V d:MoviesThe movie Mission Impossible*

V d*

Tp thc thThc thPERSONBill Clinton, ObamaPRODUCTChair, Table, MotorJOBViolinist, ProgrammerSKILL LEVELGood, Bad

Cc thuc tnh ca thc thKhi nim:Mt thuc tnh (an attribute) ca mt thc th l mt chi tit thng tin v thc th . Thuc tnh cng biu din cc thng tin cn thit cho nghip v, nhng gn lin vi thc th, ch khng ng c lp nh thc th. Mt thuc tnh c cc gi tr m thuc mt min gi tr no (kiu d liu ca n). K hiu:MoviesTitleYear of production*

V d: Cc thuc tnh*

Thuc tnh khaMoviesTitleYear of productionID*

V d Thc th, Tp thc th, Thuc tnhMoviesTitleYear of productionThe movie Mission ImpossibleTitle = Mission ImpossibleYear of production = 1996IDID = 1*

Lin kt (Relationship)Khi nim: N biu din mi quan h (rng buc) gia hai hay nhiu thc th.N cn thit cho nghip vV d:o din ch o lm PhimDin vin tham gia ng Phimo din mi Din vin tham gia ng Phim*

Lin kt (Relationship)S ngi ca lin kt:L s thc th tham gia lin kt V d: o din ch o lm Phim: lin kt 2 ngi;o din mi Din vin tham gia ng Phim: LK 3 ngi

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Lin kt (Relationship)K hiu:o din ch o lm Phim: lin kt 2 ngi;o din mi Din vin tham gia ng Phim: LK 3 ngi

MoviesDirectorsDirectsMoviesDirectorsInvites Actors*

Lin kt (Relationship)Thuc tnh ca lin kt: thng tin chi tit b sung thm cho lin kt*MoviesDirectorsDirectsDurationCost

Cc loi lin ktT lin kt (Self-relationship): l lin kt gia cc thc th ca cng mt tp thc th.Lin kt 2 ngi (Binary relationship): l lin kt gia cc thc th thuc 2 tp thc th.Lin kt Lp con (Subclass, hay cn gi l ISA): l mt loi lin kt 2 ngi c bit.Lin kt nhiu ngi (Multiway relationship): l lin kt gia cc thc th thuc t 3 tp thc th tr ln.*

V d v cc loi lin ktT lin kt

Lin kt 2 ngiPersonelder brotheryounger brotherClassStudenthasbrotherhood*

V d v cc loi lin ktLin kt ISA:PersonStudentnameaddressdobroll#schoolisa*

Lin kt nhiu ngi: Mt sinh vin tham gia mt kha hc trong mt lp hcV d v cc loi lin ktClassStudentCourseattends*

Lc lng ca Lin ktKhi nim:Lc lng (multiplicity) ca mt lin kt l s lng cc thc th ca tng tp thc th tham gia vo lin kt .Thng c 3 trng hp:Khng (0): khi c th khng c thc th no ca tp thc th tham gia lin kt, hay cng ni vic tham gia lin kt ca tp thc th ny l khng bt buc. Mt: khi c ng mt thc th ca mt tp thc th tham giaNhiu: khi c nhiu hn 1 thc th ca mt tp thc th tham gia*

V d v lc lngXt lin kt: o din ch o lm Phim:Mt o din c th ch o Khng, Mt, hoc Nhiu b phimMt b phim phi c ch o bi t nht Mt o dinMoviesDirectorsDirects1..N0..N*

Lc lng ca Lin kt 2 ngiC 3 loi lin kt 2 ngi gia hai tp thc th X v Y:Mt Mt (1:1): mi thc th trong X c quan h vi ng mt thc th trong Y v tng t ngc li. Mt Nhiu (1:N): mi thc th trong X c quan h vi nhiu thc th trong Y. Nhng ngc li mi thc th trong Y c quan h vi ng mt thc th trong X. Nhiu Nhiu (M:N): mi thc th trong X c quan h vi nhiu thc th trong Y v tng t ngc li. *

Lc lng ca Lin kt 2 ngiK hiuXY1:111XYXY1:N1NXYXYM:NMNXY*

Xy dng m hnh thc th lin ktGm cc bc:

Xc nh cc thc th v cc thuc tnh ca chngXc nh cc lin kt v cc thuc tnhHon thin m hnh

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Xc nh cc thc thPhng php:R sot trong cc Kch bn s dng, v nht l T in d liu xc nh cc n v d liu, s l cc thc th v cc thuc tnh tim nngCn nhc mc quan trng ca cc thc th v cc thuc tnh tim nng i vi h thngChn ra cc thc th v cc thuc tnh ca chngLu : cn phn bit gia cc thc th v thuc tnh.*

Xc nh cc lin ktPhng php:Xc nh cc mi quan h gia cc thc th, t xc nh ra loi lin ktXc nh lc lng ca cc thc th tham giaXc nh cc thuc tnh ca cc lin kt nu c*

Hon thin m hnhCn nhc ri thng nht vic la chn Thc th | Lin kt | Thuc tnh trong m hnhCn nhc vic b sung/loi b cc thnh phn ca m hnhTrong qu trnh ny, c th cn phi chuyn i gia Lin kt v Thc th, hoc gia Thuc tnh v Thc thV m hnh cui cng gm ton b cc thnh phn ca n*

V d: M hnh TT LK T in d liu*

Tn d liuB danhM tKhoa= Tn khoa + Vn phng + in thoi + FaxB mnBM= Tn BM + Vn phngGio vinGV=TnGV + Ngy sinh + a ch + Chc danh + (Chc v)Trng BMTBML mt gio vin c chc v l trng BMLp hcLH= Tn lp + Kha hc + S Lng SVPhng hcPHTn phng + SL ch ngiThi kha biuTKB= Nm hc + Hc k + {Khoa + {lp hc + phng hc + tit hc}m}n Khi lng ging dyKLGD= TKB + {Gio vin}mxn

V d: M hnh TT LK Cc thc th*

Tn thc thTn s dngCc thuc tnhKhoaTKhoaTn khoa, Vn phng, in thoi, FaxB mnTBoMonTn BM, Vn phngGio vinTGiaoVienTnGV, Ngy sinh, a ch, Chc danh, Chc vLp hcTLopHocTn lp, Kha hc, S Lng SVPhng hcTPhongTn phng, SL ch ngiTrng BMTTrBMGm cc thuc tnh ca Gio vin, Ngy nhm chc, Ngy thi chc

V d: M hnh TT LK Cc lin ktLin kt 1:N gia Khoa v B mn: Mt Khoa c nhiu B mn, nhng ngc li mt B mn thuc ng mt Khoa.Lin kt 1:N gia B mn v Gio vin: Mt B mn c nhiu Gio vin, nhng ngc li mt Gio vin thuc ng mt B mn.Lin kt 1:N gia Khoa v Lp hc: Mt Khoa c nhiu Lp hc, nhng ngc li mt Lp hc thuc ng mt Khoa.Lin kt ISA gia Trng B mn vi Gio vin: v Trng B mn cng l mt Gio vin.Lin kt 1:1 gia Trng B mn v B mn: mi B mn c mt Trng B mn, v ngc li mi Trng B mn lnh o mt B mn.

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V d: M hnh TT LK Cc lin ktThi kha biu l lin kt nhiu ngi gia cc thc th: Khoa, Lp hc v Phng hc. Lin kt ny s b sung thm cc thuc tnh l Nm hc, Hc k v Tit hc.Khi lng ging dy l lin kt M:N gia TKB v Gio vin: Mt TKB s phn KLGD cho nhiu nhiu Gio vin, v ngc li mt Gio vin tham gia ging dy theo nhiu TKB.*

V d: M hnh TT LK Cc lin kt*KhoaB mn1NGio vinB mnN1KhoaLp hc1NCThucCTrng B mnISALo11

V d: M hnh TT LK Cc lin kt*KhoaLp hcPhng hcTKBNm hcHc kTit hcTKBGio vinKLGDNM

V d: M hnh TT LK Hon thin*KhoaLp hcPhng hcTKBGio vinNNB mnKLGD111NNNNN1111N1Trng B mn11

Tm tt ngha ca M hnh TTLKCc thnh phn ca M hnh:Thc thLin ktCc thuc tnhCch xy dng M hnh*

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