ENTITY-RELATIONSHIP MODEL

Transcription

1 Assigned reading: Chapter 4 ENTITY-RELATIONSHIP MODEL Lecture 1

2 Relational database model Used by all major commercial database systems Very simple model Query with high-level languages: simple yet expressive Efficient implementations

3 In relational model Database = set of named relations (or tables) Each relation has a set of named attributes (or columns) Each tuple (or row) has a value for each attribute Each attribute has a type (or domain) Movies Basic Instinct Total Recall Total Recall Stars Sharon Stone Arnold Schwarzenegger Sharon Stone

4 Running example: Movies database This relation describes movies: Title Year when the movie was made Movie s length Whether the movie was in color

5 Database Design Designing a database: what information the database must hold, and what relationships are there among components of that information. Notation for expressing designs: Entity-Relationship (E/R) model Ideas E/R design Relational schema Relational DBMS Abstract design Mechanical process Concrete design

6 Elements of the E/R Model Entity - movie Movies The set of all movies constitutes an Entity set Attributes Title Relationships Stars-in

7 Example of Entity-Relationship diagram title year length filmtype name address Movies Stars-In Stars Owns Studios name address

8 Relationships Movies Owns Studios Studios Runs Presidents Movies Stars-In Stars

9 Multiplicity of Relationships many-one Movies Owns Studios Arrow says: At most one one-one Studios Runs Presidents many-many Movies Stars-In Stars

10 Sometimes binary relationships aren t enough! Example Movies Stars-In Stars Owns No arrow now! Studios What could go wrong with this design? Which stars a studio is paying for a given movie?

11 Why some studio-movie-star triples can be invalid? Carolco Pictures is paying Arnold S. for movie Total Recall and TriStar is paying Sharon S. for the same movie Total Recall. Using Owns and Stars-In we will have the following triples: (CP, TR, AS) (CP, TR, SS) (TS, TR, AS) (TS, TR, SS) The second and the third triples aren t valid. If we consider the collection of all the valid triples, it is nothing else but a three way relationship between Studios, Movies and Stars

12 Solution: Three-way relationship Stars Contracts Movies Studios

13 Attributes on Relationships title year name addr Movies Contracts Stars length filmtype Studios name addr

14 Attributes on Relationships title year salary name addr Movies Contracts Stars length filmtype Studios name addr

15 Roles in a relationship o An entity set can appear two or more times in a relationship. o Each line to the entity set represents a different role. Original Sequel-of Sequel Movies o A movie may have many sequels, but for each sequel there is only one original movie.

16 Another Example Relationship Set Married Husband Bob Joe Wife Ann Sue husband Drinkers wife

17 Another Example Relationship table Buddies 1 2 Drinkers Buddy1 Bob Joe Ann Joe Buddy2 Ann Sue Bob Moe

18 Example 1. Bars-Beer-Drinkers (BBD) Bars sell some beers. Drinkers like some beers. Drinkers frequent some bars. What would be the E/R diagram?

19 Example 1. Bars-Beer-Drinkers (BBD) name addr name manf Bars Sells Beers Bars sell some beers. license Note: license = beer, full, none Frequents name Drinkers Likes addr Drinkers like some beers. Drinkers frequent some bars. Why we need it?

20 BBD Multiway Relationship Suppose that drinkers prefer drink certain beers at certain bars.

21 BBD Multiway Relationship name addr name manf license Bars Beers Preferences Drinkers name addr

22 A Typical Relationship Table Bar Drinker Beer Joe s Bar Ann Miller Sue s Bar Ann Bud Sue s Bar Ann Pete s Ale Joe s Bar Bob Bud Joe s Bar Bob Miller Joe s Bar Cal Miller Sue s Bar Cal Bud Lite

23 Multiple Relationships Between Two Entity Sets Drinkers Likes Beers Favorite

24 Exactly one Multiplicity Manfs Bestseller Beers Some beers are not the best-seller of any manufacturer, so a rounded arrow to Manfs would be inappropriate. But a manufacturer has to have a best-seller.

25 Design exercises

26 Exercise 1. Bank database Let us design a database for a bank, including information about customers and their accounts. Information about a customer includes their name, address, phone, and SIN number. Accounts have numbers, types (e.g., savings, checking) and balances. We also need to record the customer(s) who own an account. Draw the E/R diagram for this database.

27 Exercise 1 solution name addr phone SIN number type balance Customers Owns Accounts

28 Exercise 1A. Bank database Modify your solution as follows: a) Change your diagram so an account can have only one customer.

29 Exercise 1A solution name addr phone SIN number type balance Customers Owns Accounts

30 Exercise 1B. Bank database Modify your solution as follows: a) Change your diagram so an account can have only one customer. b) Change your diagram so that a customer can have a set of addresses (which are street-city-province triples) and a set of phones. Remember that we do not allow attributes to have non-atomic types, such as sets, in the E/R model.

31 Exercise 1B solution name SIN number type balance Customers Owns Accounts Locatedat Reached-by Addresses Phones street city province number type

32 Exercise 1C. Bank database Modify your solution as follows: a) Change your diagram so an account can have only one customer. b) Change your diagram so that a customer can have a set of addresses (which are street-city-province triples) and a set of phones. c) Further modify your diagram so that customers can have a set of addresses, and at each address there is a set of phones.

33 Exercise 1C solution name SIN number type balance Customers Owns Accounts Locatedat Addresses Has Phones street city province number type

34 Exercise 2. Teams and fans database Give an E/R diagram for a database recording information about teams, players, and their fans, including: For each team, its name, its players, its team captain (one of its players), and the colors of its uniform. For each player, his/her name. For each fan, his/her name, favorite teams, favorite players, and favorite color.

35 Exercise 2 solution (Variant I) teamname Teams wears memberof captainof rootsfor Colors Players name Fans name prefers

36 Exercise 2 solution (Variant II) teamname Teams wears memberof captainof Colors name Players favors Fans name

37 Exercise 2A. Teams and fans database Modification A: Suppose we wish to add to the schema a relationship Ledby among two players and a team. The intention is that this relationship set consists of triples (player1, player2, team) such that player 1 played on the team at a time when some other player 2 was the team captain. Draw the modification to the E/R diagram.

38 Exercise 2A solution teamname wears Teams rootsfor memberof captainof leadby period Players name

39 Keys for entity sets

40 Keys A key (for an entity set) is a set of attributes such that no two entities agree on all the attributes of the key. In E/R, we underline the key attribute(s). name Beers manf

41 Keys? title year length filmtype name address Movies Stars-In Stars Owns Studios name address

42 Key for Movies Let s consider entity set Movie {title, year}

43 Key for Studios and Stars For Studios: name For Stars: name

44 Surrogate Keys Often, people introduce attributes whose role is to serve as a surrogate key. Companies assign employee ID s to all employees, and these ID s are carefully chosen to be unique numbers. In Canada everyone has a SIN. Students ID s in universities Driver license numbers Automobile registration numbers

45 From attributes to entity sets

46 Entity Sets Versus Attributes name manf manfaddr Beers Example of bad design. Why? 1. Repeats the manufacturer s address once for each beer; 2. Loses the address if there are temporarily no beers for a manufacturer.

47 When to replace attribute with an entity set An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one non-key attribute. or It is the many in a many-one or many-many relationship.

48 From attributes to entity sets Manfs deserves to be an entity set because of the non-key attribute addr. Beers deserves to be an entity set because it is the many of the many-one relationship ManfBy. name name addr Beers ManfBy Manfs

49 Exercise 2B. Teams and Fans Record for each player the history of teams on which they have played, including the start date and ending date (if they were traded) for each such team.

50 Inheritance in the ER model

51 Subclasses Sometimes, an entity set contains certain entities that have special properties not associated with all members of this entity set. In this case it is useful to define special-case entity sets, or subclasses, each with its own attributes and relationships

52 Subclasses to Stars length title year filmtype Movies Relate parent with child by a special (1-1) relationship called isa. Voices isa isa weapon Cartoons Murder- Mysteries

53 Inheritance in the E/R Model In the object-oriented world, property values are in one class only. Subclasses inherit the property definition from superclasses. In contrast, E/R entities participate in all subclasses to which they belong.

54 Example Roger Rabbit, which is both a cartoon and murdermystery will have one tuple in each of all three entity sets: Movies, Cartoons, and Murder-Mysteries. i.e. it will have all four attributes of Movies, the attribute weapon, and finally will participate in the relationship voices. to Stars length title year filmtype Movies Voices isa isa weapon Cartoons Murder- Mysteries

55 Keys for entity set hierarchies In entity set hierarchies the key at root is key for all. {title,year} is the key for Movies, Cartoons and Murder-Mysteries. to Stars length title year filmtype Movies Voices isa isa weapon Cartoons Murder- Mysteries

56 Exercise 3 Suppose we wish to keep a genealogy. We shall have one entity set, Person. The information we wish to record about persons includes their name (an attribute) and the following relationships: mother, father, and children. Give an E/R diagram involving the Person entity set and all the relationships in which it is involved. Include relationships for mother, father, and children. Modify your people" database design to include the following special types of people: a) Females. b) Males. c) People who are parents.

57 Week entity sets

58 Week entity sets It is possible that the key of an entity set is composed of attributes, some or all of which do not belong to this entity set Such an entity set is called a week entity set We use week entity sets to identify sub-units of the main entity, rather than sub-classes

59 Supporting relationships In a week entity set E the key consists of: Zero or more its own attributes Keys from other entities reached by many-one relationship from E These relationships are called supporting relationships

60 Example of a weak entity set number role name address Crews Unit-of Studios No. of emp. o o o E.g. Crew 1, Special Effects for Paramount, Crew 1, Special Effects for Fox, Need to add the key for Studios, in order to have a key for Crews. Crews is a weak entity set.

61 In E/R diagrams number role name address Crews Unit-of Studios No. of emp. o Double rectangle for the weak entity set. o Double diamond for a supporting many-one relationship.

62 Another Example Football Players name is almost a key for football players, but there might be two with the same name. number is certainly not a key, since players on two teams could have the same number. But number, together with the team name related to the player by Plays-on should be unique. name number name Players Playson Teams

63 Another Example Football Players name number name Players Playson Teams

64 Supporting vs. regular relationships Not all the many-one relationships connecting a weak entity set to other entity sets are supporting relationships. E.g. number role name address Operat. Cities Crews Unit-of Studios Op No. of emp.

65 When Do We Need Weak Entity Sets? o Usual reason: no global authority capable of creating unique ID s (surrogate key). o E.g.: Unlikely there could be an agreement to assign unique player numbers across all football teams in the world.