1 Entity-Relationship Model Slides by Jeffrey Ullman Modified by J. Welch to replace beers with candies

2 Purpose of E/R Model uThe E/R model allows us to sketch database designs. wKinds of data and how they connect. wNot how data changes. uDesigns are pictures called entity- relationship diagrams. uLater: convert E/R designs to relational DB designs.

3 Entity Sets uEntity = “thing” or object. uEntity set = collection of similar entities. wSimilar to a class in object-oriented languages. uAttribute = property of (the entities of) an entity set. wAttributes are simple values, e.g. integers or character strings.

4 E/R Diagrams uIn an entity-relationship diagram: wEntity set = rectangle. wAttribute = oval, with a line to the rectangle representing its entity set.

5 Example uEntity set Candies has two attributes, name and manf (manufacturer). uEach Candies entity has values for these two attributes, e.g. (Twizzlers, Hershey) Candies name manf

6 Relationships uA relationship connects two or more entity sets. uIt is represented by a diamond, with lines to each of the entity sets involved.

7 Example Con- sumers addrname Candies manfname Stores name license addr Note: license = beer, full, none Sells Stores sell some candies. Likes Consumers like some candies. Frequents Consumers frequent some stores.

8 Relationship Set uThe current “value” of an entity set is the set of entities that belong to it. wExample: the set of all stores in our database. uThe “value” of a relationship is a set of lists of currently related entities, one from each of the related entity sets.

9 Example uFor the relationship Sells, we might have a relationship set like: StoreCandy 7-11Twizzlers 7-11Kitkat KrogerTwizzlers KrogerSnickers KrogerReeses

10 Multiway Relationships uSometimes, we need a relationship that connects more than two entity sets. uSuppose that consumers will only buy certain candies at certain stores. wOur three binary relationships Likes, Sells, and Frequents do not allow us to make this distinction. wBut a 3-way relationship would.

11 Example Stores Candies Con- sumers name addr manf nameaddr license Preferences

12 A Typical Relationship Set StoreConsumerCandy 7-11AnnKitkat KrogerAnnTwizzler KrogerAnnSnickers 7-11BobTwizzler 7-11BobKitkat 7-11CalKitkat KrogerCalReeses

13 Many-Many Relationships uFocus: binary relationships, such as Sells between Stores and Candies. uIn a many-many relationship, an entity of either set can be connected to many entities of the other set. wE.g., a store sells many candies; a candy is sold by many stores.

14 In Pictures: many-many

15 Many-One Relationships uSome binary relationships are many - one from one entity set to another. uEach entity of the first set is connected to at most one entity of the second set. uBut an entity of the second set can be connected to zero, one, or many entities of the first set.

16 In Pictures: many-one

17 Example uFavorite, from Consumers to Candies is many-one. uA consumer has at most one favorite candy. uBut a candy can be the favorite of any number of consumers, including zero.

18 One-One Relationships uIn a one-one relationship, each entity of either entity set is related to at most one entity of the other set. uExample: Relationship Best-seller between entity sets Manfs (manufacturer) and Candies. wA candy cannot be made by more than one manufacturer, and no manufacturer can have more than one best-seller (assume no ties).

19 In Pictures: one-one

20 Representing “Multiplicity” uShow a many-one relationship by an arrow entering the “one” side. uShow a one-one relationship by arrows entering both entity sets. uRounded arrow = “exactly one,” i.e., each entity of the first set is related to exactly one entity of the target set.

21 Example Con- sumers Candies Likes Favorite

22 Example uConsider Best-seller between Manfs and Candies. uSome candies are not the best-seller of any manufacturer, so a rounded arrow to Manfs would be inappropriate. uBut a candy manufacturer has to have a best-seller.

23 In the E/R Diagram ManfsCandies Best- seller

24 Attributes on Relationships uSometimes it is useful to attach an attribute to a relationship. uThink of this attribute as a property of tuples in the relationship set.

25 Example StoresCandies Sells price Price is a function of both the candy and the store, not of one alone.

26 Equivalent Diagrams Without Attributes on Relationships uCreate an entity set representing values of the attribute. uMake that entity set participate in the relationship.

27 Example StoresCandies Sells price Prices Note convention: arrow from multiway relationship = “all other entity sets together determine a unique one of these.”

28 Roles uSometimes an entity set appears more than once in a relationship. uLabel the edges between the relationship and the entity set with names called roles.

29 Example Con- sumers Married husbandwife Relationship Set HusbandWife BobAnn JoeSue…

30 Example Con- sumers Buddies 12 Relationship Set Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe …

31 Subclasses uSubclass = special case = fewer entities = more properties. uExample: Chocolates are a kind of candy. wNot every candy is a chocolate, but some are. wLet us suppose that in addition to all the properties (attributes and relationships) of candies, chocolates also have the attribute color (dark, white or light).

32 Subclasses in E/R Diagrams uAssume subclasses form a tree. wI.e., no multiple inheritance. uIsa triangles indicate the subclass relationship. wPoint to the superclass.

33 Example Candies Choc- olates isa namemanf color

34 E/R Vs. Object-Oriented Subclasses uIn OO, objects are in one class only. wSubclasses inherit from superclasses. uIn contrast, E/R entities have representatives in all subclasses to which they belong. wRule: if entity e is represented in a subclass, then e is represented in the superclass.

35 Example Candies Choc- olates isa namemanf color Snickers

36 Keys uA key is a set of attributes for one entity set such that no two entities in this set agree on all the attributes of the key. wIt is allowed for two entities to agree on some, but not all, of the key attributes. uWe must designate a key for every entity set.

37 Keys in E/R Diagrams uUnderline the key attribute(s). uIn an Isa hierarchy, only the root entity set has a key, and it must serve as the key for all entities in the hierarchy.

38 Example: name is Key for Candies Candies Choc- olates isa namemanf color

39 Example: a Multi-attribute Key Courses dept number hoursroom Note that hours and room could also serve as a key, but we must select only one key.

40 Weak Entity Sets uOccasionally, entities of an entity set need “help” to identify them uniquely. uEntity set E is said to be weak if in order to identify entities of E uniquely, we need to follow one or more many- one relationships from E and include the key of the related entities from the connected entity sets.

41 Example uname is almost a key for football players, but there might be two with the same name. unumber is certainly not a key, since players on two teams could have the same number. uBut number, together with the team name related to the player by Plays-on should be unique.

42 In E/R Diagrams PlayersTeams Plays- on name number Double diamond for supporting many-one relationship. Double rectangle for the weak entity set.

43 Weak Entity-Set Rules uA weak entity set has one or more many-one relationships to other (supporting) entity sets. wNot every many-one relationship from a weak entity set need be supporting. uThe key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets. wE.g., (player) number and (team) name is a key for Players in the previous example.

44 Design Techniques 1.Avoid redundancy. 2.Limit the use of weak entity sets. 3.Don’t use an entity set when an attribute will do.

45 Avoiding Redundancy uRedundancy occurs when we say the same thing in two or more different ways. uRedundancy wastes space and (more importantly) encourages inconsistency. wThe two instances of the same fact may become inconsistent if we change one and forget to change the other.

46 Example: Good CandiesManfs ManfBy name This design gives the address of each manufacturer exactly once. nameaddr

47 Example: Bad CandiesManfs ManfBy name This design states the manufacturer of a candy twice: as an attribute and as a related entity. name manf addr

48 Example: Bad Candies name This design  repeats the manufacturer’s address once for each candy and  loses the address if there are temporarily no candies for a manufacturer. manfmanfAddr

49 Entity Sets Versus Attributes uAn entity set should satisfy at least one of the following conditions: wIt is more than the name of something; it has at least one nonkey attribute. or wIt is the “many” in a many-one or many- many relationship.

50 Example: Good CandiesManfs ManfBy name Manfs deserves to be an entity set because of the nonkey attribute addr. Candies deserves to be an entity set because it is the “many” of the many-one relationship ManfBy. nameaddr

51 Example: Good Candies name There is no need to make the manufacturer an entity set, because we record nothing about manufacturers besides their name. manf

52 Example: Bad CandiesManfs ManfBy name Since the manufacturer is nothing but a name, and is not at the “many” end of any relationship, it should not be an entity set. name

53 Don’t Overuse Weak Entity Sets uBeginning database designers often doubt that anything could be a key by itself. wThey make all entity sets weak, supported by all other entity sets to which they are linked. uIn reality, we usually create unique ID’s for entity sets. wExamples include social-security numbers, automobile VIN’s etc.

54 When Do We Need Weak Entity Sets? uThe usual reason is that there is no global authority capable of creating unique ID’s. uExample: it is unlikely that there could be an agreement to assign unique player numbers across all football teams in the world.