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Slide 53 -
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1 Entity-Relationship Model Slides by Jeffrey Ullman
Modified by J. Welch to replace beers with candies 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 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 E/R Diagrams In an entity-relationship diagram:
Entity set = rectangle.
Attribute = oval, with a line to the rectangle representing its entity set. 5 Example
Entity set Candies has two attributes, name and manf (manufacturer).
Each Candies entity has values for these two attributes, e.g. (Twizzlers, Hershey) 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 Example Stores name addr Consumers frequent
some stores. 8 Relationship Set The current “value” of an entity set is the set of entities that belong to it.
Example: the set of all stores 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 Example For the relationship Sells, we might have a relationship set like: Store Candy
7-11 Twizzlers
7-11 Kitkat
Kroger Twizzlers
Kroger Snickers
Kroger Reeses 10 Multiway Relationships Sometimes, we need a relationship that connects more than two entity sets.
Suppose that consumers will only buy certain candies at certain stores.
Our three binary relationships Likes, Sells, and Frequents do not allow us to make this distinction.
But a 3-way relationship would. 11 Example Stores Candies Con-
sumers name name addr manf name addr license Preferences 12 A Typical Relationship Set Store Consumer Candy
7-11 Ann Kitkat
Kroger Ann Twizzler
Kroger Ann Snickers
7-11 Bob Twizzler
7-11 Bob Kitkat
7-11 Cal Kitkat
Kroger Cal Reeses 13 Many-Many Relationships Focus: binary relationships, such as Sells between Stores and Candies.
In a many-many relationship, an entity of either set can be connected to many entities of the other set.
E.g., a store sells many candies; a candy is sold by many stores. 14 In Pictures: many-many 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 In Pictures: many-one 17 Example Favorite, from Consumers to Candies is many-one.
A consumer has at most one favorite candy.
But a candy can be the favorite of any number of consumers, including zero. 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 Candies.
A 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” 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 Example Con-
sumers Candies Likes Favorite 22 Example Consider Best-seller between Manfs and Candies.
Some candies are not the best-seller of any manufacturer, so a rounded arrow to Manfs would be inappropriate.
But a candy manufacturer has to have a best-seller. 23 In the E/R Diagram Manfs Candies Best-
seller 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 Example Stores Candies Sells price Price is a function of both the candy and the store, not of one alone. 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 Example Stores Candies Sells price Prices Note convention: arrow
from multiway relationship
= “all other entity sets
together determine a
unique one of these.” 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 Example Con-
sumers Married husband wife 30 Example Con-
sumers Buddies 1 2 Relationship Set
Buddy1 Buddy2
Bob Ann
Joe Sue
Ann Bob
Joe Moe
… … 31 Subclasses Subclass = special case = fewer entities = more properties.
Example: Chocolates are a kind of candy.
Not every candy is a chocolate, but some are.
Let 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 Assume subclasses form a tree.
I.e., no multiple inheritance.
Isa triangles indicate the subclass relationship.
Point to the superclass. 33 Example Candies Choc-
olates isa name manf color 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 Example Candies Choc-
olates isa name manf color 36 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. 37 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. 38 Example: name is Key for Candies Candies Choc-
olates isa name manf color 39 Example: a Multi-attribute Key Courses dept number hours room Note that hours and room could also serve as a
key, but we must select only one key. 40 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. 41 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. 42 In E/R Diagrams Players Teams Plays-
on name name number Double diamond for supporting many-one relationship.
Double rectangle for the weak entity set. 43 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. 44 Design Techniques Avoid redundancy.
Limit the use of weak entity sets.
Don’t use an entity set when an attribute will do. 45 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. 46 Example: Good Candies Manfs ManfBy name This design gives the address of each manufacturer exactly once. name addr 47 Example: Bad Candies Manfs 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. manf manfAddr 49 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. 50 Example: Good Candies Manfs 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. name addr 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 Candies Manfs 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 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.
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