Relational Model. Lecture #6 Autumn, Fall, 2001, LRX

Transcription

1 Reationa Mode Lecture #6 Autumn, 2001 #06 Reationa Mode HUST,Wuhan,China 121

2 Reationa Mode Tabe = reation. Coumn headers = attributes. Row = tupe Reation schema = name(attributes). Exampe: Beers(name, manf).» Order of attributes is arbitrary, but in practice we need to assume the order given in the reation schema.» Reation instance is current set of rows for a reation schema. Database schema = coection of reation schemas. #06 Reationa Mode HUST,Wuhan,China 122

3 Reationa Mode (II) #06 Reationa Mode HUST,Wuhan,China 123

4 Keys in Reations An attribute or set of attributes K is a key for a reation R if we expect that in no instance of R wi two different tupes agree on a the attributes of K. Indicate a key by underining the key attributes.» Exampe: If name is a key for Beers: Beers(name, manf) #06 Reationa Mode HUST,Wuhan,China 124

5 Why Reations? Very simpe mode. Often a good match for the way we think about our data. Abstract mode that underies SQL, the most important anguage in DBMS's today.» And even infuentia in competitors ike OQL. #06 Reationa Mode HUST,Wuhan,China 125

6 Abstract Vs. Concrete Reations The reationa mode impemented in SQL differs sighty from the abstract notion of reations that we sha earn first. Big difference: abstract reations are sets of tupes; SQL reations are bags of tupes (i.e., dupicates aowed). Abstract reations vita for foundation:» Semantics of SQL statements.» Forma meaning of functiona dependencies, normaization. #06 Reationa Mode HUST,Wuhan,China 126

7 Reationa Design Reations are coser to rea storage structures than the concepts of E/R or ODL.» Thus, going from E/R or ODL designs to reationa often requires some additiona inteectua input. Easiest Case: Entity Set -> Reation» E. S. attributes become reationa attributes. #06 Reationa Mode HUST,Wuhan,China 127

8 Reationa Design (II) #06 Reationa Mode HUST,Wuhan,China 128

9 Sighty Harder: ODL Cass Without Reationships Probem: ODL aows attribute types buid from structures and coection types. Structure: Make one attribute for each fied. Set: make one tupe for each member of the set.» More than one set attribute? Make tupes for a combinations. Probem: ODL cass may have no key, but we shoud have one in the reation to represent OID." #06 Reationa Mode HUST,Wuhan,China 129

10 Exampe interface Drinkers (key name) { attribute string name; attribute Struct Addr { string street, string city, int zip} address; attribute Set<string> phone; } #06 Reationa Mode HUST,Wuhan,China 130

11 Exampe (II) name street city zip phone n1 s1 c1 z1 p1 n1 s1 c1 z1 p2 #06 Reationa Mode HUST,Wuhan,China 131

12 Exampe (III) Surprise: the key for the cass (name) is not the key for the reation (name, phone).» name in the cass determines a unique object, incuding a unique set of phones.» name in the reation does not determine a unique tupe.» Since tupes are not identica to objects, there is no inconsistency! #06 Reationa Mode HUST,Wuhan,China 132

13 Decompose Reations? One option is to get phone into a separate reation (with name). The database woud ook ike: #06 Reationa Mode HUST,Wuhan,China 133

14 Decompose Reations? (II) #06 Reationa Mode HUST,Wuhan,China 134

15 Decompose Reations? (II) Advantages:»1. Avoids redundancy in address components.»2. Handes the case where someone has no phone. Disadvantage:»Harder to answer queries that jump between two reations, e.g., in what city is phone ? #06 Reationa Mode HUST,Wuhan,China 135

16 A Design Probem interface Famiy { attribute Set<string> parents; attribute Set<string> chidren; } 1. What is the key? 2. How shoud we represent a famiy with two parents and three chidren? 3. Woud you favor decomposition into severa reations? #06 Reationa Mode HUST,Wuhan,China 136

17 E/R Reationships -> Reations Reation has attribute for key attributes of each E.S. that participates in the reationship. Key of reation excudes attributes from the one side if reationship is many-one. For a one-one reationship, choose which side provides the key of the reation. Renaming attributes OK.» Essentia if mutipe roes for an E.S. #06 Reationa Mode HUST,Wuhan,China 137

18 E/R Reationships -> Reations (II) #06 Reationa Mode HUST,Wuhan,China 138

19 E/R Reationships -> Reations (III) Likes(drinker, beer) Favorite(drinker, beer) Married(husband, wife) Buddies(name1, name2) For one-one reation Married, we can choose either husband or wife as key. #06 Reationa Mode HUST,Wuhan,China 139

20 Weak Entity Sets, Reationships -> Reations Hosts(hostName) Logins(oginName, hostname) At(oginName, hostname, hostname2) In At, hostname and hostname2 must be the same host, so deete one of them. Then, Logins and At become the same reation; deete one of them. In this case, Hosts' schema is a subset of Logins' schema. Deete Hosts? Genera rue: Deete the reation that comes from a manyone reationship supporting a weak entity set. #06 Reationa Mode HUST,Wuhan,China 140

21 ODL Reationships Pick one direction, say A -> B. Put key of B attributes in the reation for cass A. Prefer to make A the many," if reationship is many-one. If reationship is many-many, we' have to dupicate A-tupes as in E/R. #06 Reationa Mode HUST,Wuhan,China 141

22 Exampe interface Drinkers { attribute string name; attribute string addr; reationship Set<Beers> ikes inverse Beers::fans; reationship Beers favorite inverse Beers::reaFans; reationship Drinkers husband inverse wife; reationship Drinkers wife inverse husband; reationship Set<Drinkers> buddies inverse buddies; } #06 Reationa Mode HUST,Wuhan,China 142

23 Exampe (II) Drinkers(name, addr, wifename, buddyname, beername, favoritebeer) #06 Reationa Mode HUST,Wuhan,China 143

24 Subcasses -> Reations Three approaches:» 1. ODL stye: each object is in one cass. Create a reation for each cass, with a the attributes for that cass. Don't forget inherited attributes.» 2. E/R stye: an entity is in a network of casses reated by isa. Create one reation for each E.S.; entities represented in a E.S. to which it beongs.» Reation has ony the attributes attached to that E.S. + key. #06 Reationa Mode HUST,Wuhan,China 144

25 Subcasses -> Reations (II)» 3. Use nus. Create one reation for the root cass or root E.S., with a attributes found anywhere in its network of subcasses.» Put NULL in attributes not reevant to an object/entity. #06 Reationa Mode HUST,Wuhan,China 145

26 Exampe name Beers manf isa coor Aes #06 Reationa Mode HUST,Wuhan,China 146

27 Exampe (II) interface Beers (key name) { attribute string name; attribute string manf; } interface Aes:Beers { attribute string coor; } #06 Reationa Mode HUST,Wuhan,China 147

28 ODL Stye Name Bud Beers manf A.B. Name manf coor Summerbrew Pete s dark #06 Reationa Mode HUST,Wuhan,China 148

29 E/R Stye Name Bud SummerBrew Beers manf A.B. Pete s Name SummerBrew coor dark Aes #06 Reationa Mode HUST,Wuhan,China 149

30 Using Nus Name manf coor Bud A.B. NULL SummerBrew Pete s dark Beers #06 Reationa Mode HUST,Wuhan,China 150

31 Design Chaenge Remember the probem with oca and express trains and oca and express stations?» Trains have numbers and engineers.» Stations have names and addresses.» There is a time for each train/station pair where a stop occurs. How woud we best represent this information as reations? Can we design the database schema so it is impossibe for an express train to have a stop time for a oca station? Shoud we? #06 Reationa Mode HUST,Wuhan,China 151