Chng 5

M hnh d liu quan h

Phn 1

Ni dung chnhGii thiu v M hnh d liuTng quan v M hnh d liu quan h*/192

Tng quan v M hnh d liu*/19

M hnh d liu l g? M hnh d liu (Data Model) l cch k hiu m t d liu hay thng tin. Ni chung, n gm c ba phn:Cu trc ca d liu (Structure of the data)Cc thao tc trn d liu (Operations on data)Cc rng buc (Constraints)*/19

Tm tt lch s ca cc M hnh d liu*/19

Tm tt lch s ca cc M hnh d liu*/19

Tng quan v M hnh d liu quan h*/19

Quan h l g?Quan h (relation) l mt bng hai chiu:Cc ct gi l cc thuc tnh (attribute)Cc hng gi l cc bn ghi (record) hay b (tuple)*/19

Quan h l g?V d: bng Customers:Mi hng biu din cho mt CUSTOMERMi ct biu din cho mt thuc tnh ca CUSTOMER v c gi l thuc tnh

*/19

Lc (Schema)Lc ca mt quan h l thng tin v cu trc ca quan h N bao gm tn v tp cc thuc tnh ca quan h , cng mt s thng tin rng buc ca cc thuc tnh. V d: Lc ca quan h Customers trn:Customers(Customers ID, tax ID, name, address)*/19

B v thnh phnMt hng ca mt quan h c gi l mt b (tuple) hay bn ghi (record)Mt gi tr ca mt thuc tnh trong mt b c gi l mt thnh phn (component)*/19

Min gi tr (Domains)Min gi tr ca mt thuc tnh l tp tt c cc gi tr m thuc tnh c th nhnNn khi nim min gi tr tng ng vi khi nim kiu d liu (data type) mc ci t trong mt h qun tr CSDL, th min gi tr ca cc thuc tnh ca mt quan h cn phi chn sao cho nm trong s cc kiu d liu m h QT CSDL h tr*/19

Biu din tng ng ca quan hQuan h l tp hp ca cc b (sets of tuples), ch khng phi danh sch ca cc b (lists of tuples).Do vy, th t ca cc b trong quan h KHNG quan trng, m quan trng l ni dung ca cc b. ng thi nu c 2 b c gi tr bng nhau trn mi thuc tnh th coi nh 1 b b tha.*/19

Th hin ca quan hTh hin ca quan h (instance of a relation) l tp cc b ca quan h ti mt thi imTh hin ca mt quan h phn nh ni dung ca quan h , v n thay i khi c cc thao tc cp nht ni dung quan h nh thm, sa hay xa b. */19

Kha ca quan hKha (key) ca mt quan h r l mt hoc mt tp hp cc thuc tnh ca r, sao cho vi mt th hin ca r, th khng tn ti 2 b no m gi tr ca cc thnh phn bng nhau trn kha. Hay ni cch khc, gi tr ca cc thnh phn trn Kha ca mi b trong mt th hin ca quan h r l duy nht.

*/19

Kha ca quan hC hai loi Kha ca quan hKha ca mt th hin ca quan hKha ca lc ca quan h: l kha ca mi th hin ca QH VD: vi th hin s1 ca quan h Student sau:

*/19C cc Kha l:K1 = ID_SVK2 = NameK3 = Address

ID_SVNameAddressClass20091111Nguyn Vn AH niT120091112Nguyn Hng BQung ninhT220091113Trn L CH tyT120091114V Th DBc giangT2

Kha ca quan hVD: nhng vi th hin s2 ca quan h Student sau:

th ch cn mt Kha l:K1 = ID_SV

K1 = ID_SV l Kha ca lc Sinh vin,Cn K2 v K3 ch l Kha ca th hin s1

*/19

ID_SVNameAddressClass20091111Nguyn Vn AH niT120091112Nguyn Hng BQung ninhT220091113Nguyn Vn AH tyT120091114V Th DH niT2

Kha ca quan hTnh cht:Kha ca th hin th cha chc l Kha ca lc Cn Kha ca lc th cng l Kha ca mi th hin ca quan hKha ca lc khng ph thuc vo ni dung ca quan h, cn kha ca th hin th ph thuc*/19

C s d liu quan hCSDL quan h (Relational Database): L tp hp cc quan h v cc rng buc gia chng, m c lin quan n mt h thng thng tin (phn mm) no */19

Tm ttQuan h = bng.Ct ca bng = thuc tnh ca quan h.B = hng = bn ghiLc QH = tn + tp cc thuc tnhTh hin ca QH = ni dung ca QH ti mt thi imKha ca QH = tp con cc thuc tnh ca QHKha ca th hinKha ca lc C s d liu quan h = tp cc quan h*/19

*Data Model: M hnh d liuConstraint: rng buc v d liu, ngm th hin cho cc quy tc nghip v

Attribute: Thuc tnh*Schema: lc *Tuple: b hay bn ghi*