1. 1 Entity-Relationship Model Diagrams Class hierarchies Weak entity sets

2. 2 Purpose of E/R Model The E/R model allows us to sketch database designs.  Kinds of data and how they connect.  Not how data changes. Designs are pictures called entity- relationship diagrams. Later: convert E/R designs to relational DB designs.

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

4. 4 E/R Diagrams In an entity-relationship diagram:  Entity set = rectangle.  Attribute = oval, with a line to the rectangle representing its entity set.

5. 5 Example Entity set Beers has two attributes, name and manf (manufacturer). Each Beers entity has values for these two attributes, e.g. (Bud, Anheuser-Busch) Beers name manf

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

7. 7 Example Drinkers addrname Beers manfname Bars name license addr Note: license = beer, full, none Sells Bars sell some beers. Likes Drinkers like some beers. Frequents Drinkers frequent some bars.

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

9. 9 Example For the relationship Sells, we might have a relationship set like: BarBeer Joe’s BarBud Joe’s BarMiller Sue’s BarBud Sue’s BarPete’s Ale Sue’s BarBud Lite

10. 10 Multiway Relationships Sometimes, we need a relationship that connects more than two entity sets. Suppose that drinkers will only drink certain beers at certain bars.  Our three binary relationships Likes, Sells, and Frequents do not allow us to make this distinction.  But a 3-way relationship would.

11. 11 Example Bars Beers Drinkers name addr manf nameaddr license Preferences

12. 12 A Typical Relationship Set BarDrinkerBeer Joe’s BarAnnMiller Sue’s BarAnnBud Sue’s BarAnnPete’s Ale Joe’s BarBobBud Joe’s BarBobMiller Joe’s BarCalMiller Sue’s BarCalBud Lite

13. 13 Many-Many Relationships Focus: binary relationships, such as Sells between Bars and Beers. In a many-many relationship, an entity of either set can be connected to many entities of the other set.  E.g., a bar sells many beers; a beer is sold by many bars.

14. 14 In Pictures: many-many

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

16. 16 In Pictures: many-one

17. 17 Example Favorite, from Drinkers to Beers is many-one. A drinker has at most one favorite beer. But a beer can be the favorite of any number of drinkers, including zero.

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

19. 19 In Pictures: one-one

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

21. 21 Example DrinkersBeers Likes Favorite

22. 22 Example Consider Best-seller between Manfs and Beers. Some beers are not the best-seller of any manufacturer, so a rounded arrow to Manfs would be inappropriate. But a beer manufacturer has to have a best-seller.

23. 23 In the E/R Diagram ManfsBeers Best- seller Can you come up with another example ?

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

25. 25 Example BarsBeers Sells price Price is a function of both the bar and the beer, not of one alone. Any other example?

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

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

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

29. 29 Example Drinkers Married husbandwife Relationship Set HusbandWife BobAnn JoeSue…

30. 30 Example Drinkers Buddies 12 Relationship Set Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe … Can you give an example of many-to-one relationship from the same entity set?

31. 31 Subclasses Subclass = special case = fewer entities = more properties. Example: Ales are a kind of beer.  Not every beer is an ale, but some are.  Let us suppose that in addition to all the properties (attributes and relationships) of beers, ales also have the attribute color.

32. 32 Subclasses in E/R Diagrams Assume subclasses form a tree.  I.e., no multiple inheritance. Isa triangles indicate the subclass relationship.  Point to the superclass.

33. 33 Example Beers Ales isa namemanf color

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

35. 35 Example Beers Ales isa namemanf color Pete’s Ale

36. 36 Subclass/Superclass Relationships Reason: An ES may have members with special properties not associated with all ES members. Example: Different accounts have different attributes.  Checking Account: overdraft amount,  Savings account: interest-rate. Possible representations in ER:  Add an attribute “accountType”: a checking account has a value for the “overdraft” attribute. A savings account has a value for the “rate” attribute. Problem: inconsistency; useless attributes; different accounts participate in different relationships.  Use 3 ES’s: checking, savings, and accounts. Problems: Not intuitive: checking and savings are really accounts! Redundancy: e.g., savings info stored in two ES’s.

37. 37 Subclass/Superclass Relationships accounts ISA savings checkings account# balance rate overdraft “Savings” and “checkings” are subclasses of the “account” ES. An entity in a subclass must belong to the superclass as well.  Every savings/checking account is also an account. Attribute Inheritance:  Subclasses inherit all attributes of the superclass.  Key of the subclass is the same as the key for the superclass.  Subclasses inherit all relationships in which the superclass participates.

38. 38 Keys A 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.  It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for every entity set.

39. 39 Keys in E/R Diagrams Underline the key attribute(s). In an Isa hierarchy, only the root entity set has a key, and it must serve as the key for all entities in the hierarchy.

40. 40 Example: name is Key for Beers Beers Ales isa namemanf color

41. 41 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.

42. 42 Modeling of Constraints Keys Single-value constraints Referential integrity  Some value referred to by some object exists Domain constraints  Value of attributes must be drawn from certain set or range General constraints  Arbitrarily assertions specified by user  E.g., no more than 10 stars listed for any one movie

43. 43 Single-Value Constraint Each attribute of an entity set has a single value  Sometimes OK to have an attribute’s value missing for some entities -> “null value”  E.g., the length of some movies unknown Many-one relationship implies a single- value constraint

44. 44 Referential Integrity Exactly-one relationship Enforcing the constraint  Forbid the deletion of a referenced entity E.g., cannot delete a bestseller beer  If a referenced entity deleted, delete all entities that reference it

45. 45 Referential integrity constraints Every customer has exactly one account Represented as a rounded arrow entering “Account” Same as: customer custacct account opendate customer custacct account opendate

46. 46 Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely. Entity 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.

47. 47 Example 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.

48. 48 In E/R Diagrams PlayersTeams Plays- on name number Double diamond for supporting many-one relationship. Double rectangle for the weak entity set.

49. 49 Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets.  Not every many-one relationship from a weak entity set need be supporting. The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets.  E.g., (player) number and (team) name is a key for Players in the previous example. Must satisfy the referential integrity from the weak entity set to the supporting entity sets

50. 50 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.

51. 51 Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency.  The two instances of the same fact may become inconsistent if we change one and forget to change the other.

52. 52 Example: Good BeersManfs ManfBy name This design gives the address of each manufacturer exactly once. nameaddr

53. 53 Example: Bad BeersManfs ManfBy name This design states the manufacturer of a beer twice: as an attribute and as a related entity. name manf addr

54. 54 Example: Bad Beers name This design repeats the manufacturer’s address once for each beer and loses the address if there are temporarily no beers for a manufacturer. manfmanfAddr

55. 55 Entity Sets Versus Attributes 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 nonkey attribute. or  It is the “many” in a many-one or many- many relationship.

56. 56 Example: Good BeersManfs ManfBy name Manfs deserves to be an entity set because of the nonkey attribute addr. Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy. nameaddr

57. 57 Example: Good Beers name There is no need to make the manufacturer an entity set, because we record nothing about manufacturers besides their name. manf

58. 58 Example: Bad BeersManfs 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

59. 59 Don’t Overuse Weak Entity Sets Beginning database designers often doubt that anything could be a key by itself.  They make all entity sets weak, supported by all other entity sets to which they are linked. In reality, we usually create unique ID’s for entity sets.  Examples include social-security numbers, automobile VIN’s etc.

60. 60 When Do We Need Weak Entity Sets? The usual reason is that there is no global authority capable of creating unique ID’s. Example: it is unlikely that there could be an agreement to assign unique player numbers across all football teams in the world.

61. 61 Case Study 1 Design a DB representing cities, counties, and states in the US :  For states, record the name, population, and state capital (a city).  For counties, record the name, the population, and the located state.  For cities, record the name, the population, the located state and the located county. Uniqueness assumptions:  Names of states are unique.  Names of counties are unique within a state (e.g., 26 states have Washington Counties).  Cities are unique only within a state (e.g., there are 24 Springfields among the 50 states).  Some counties and cities have the same name, even within a state (e.g., Los Angeles).  All cities are located within a single county

62. 62 Design 1: bad cities states Popu. name Co. Popu. Co. name Located capit al Ci. Popu. Ci. name Problem: County Population is repeated for each city.

63. 63 Design 2: good cities counties states Popu. name Located Co. Popu. Co. name capitals Ci. Popu. Ci. name Belongs-to

64. 64 Case Study 2 Design a DB consistent with the following facts.  Trains are either local trains or express trains, but never both.  A train has a unique number and an engineer.  Stations are either express stops or local stops, but never both.  A station has a unique name and an address.  All local trains stop at all stations.  Express trains stop only at express stations.  For each train and each station the train stops at, there is a time.

65. 65 Design 1: bad Problem: does not capture the constraints that express trains only stop only at express stations and local trains stop at all local stations trains stations StopsAt type number time name engineer addr type

66. 66 Design 2: good train express trains local trains stations express stations local stations name address numberengineer StopsAt1 StopsAt2 time ISA