CS 320: Database Management Systems

Entity-Relationship Model

Ge (Frank) Xia, CS, Lafayette College

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.

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.

E/R Diagrams

• In an entity-relationship diagram:

» Entity set = rectangle.

» Attribute = oval, with a line to the rectangle representing its entity set.

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)

Relationships

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

Example

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.

Example

• For the relationship Sells, we might have a relationship set like:

Bar	Beer
Joe's Bar Joe's Bar Sue's Bar Sue's Bar Sue's Bar	Bud Miller Bud Pete’s Ale Bud Lite

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.

Example

A Typical Relationship Set

Bar	Drinker	Beer
Joe's Bar Sue's Bar Sue's Bar Joe's Bar Joe's Bar Joe's Bar Sue's Bar Sue's Bar	Ann Ann Ann Bob Bob Bob Carl Carl	Bud Miller Bud Pete’s Ale Bud Lite Miller Miller Bud Lite

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.

In Pictures

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.

In Pictures

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

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).

In Pictures

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.

Example

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.

In the E/R Diagram

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.

Example

Equivalent Diagrams

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

Example

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.

Example

Example

Subclass

• 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.

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.

Example

E/R Vs. OO Subclasses

• In Object-Oriented Subclasses, 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.

Example

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.

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.

Example

• name is Key for Beers

Example

• a Multi-attribute Key

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.

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.

In E/R Diagrams

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.

Design Techniques

• Avoid redundancy.
• Limit the use of weak entity sets.
• Don’t use an entity set when an attribute will do.

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.

Example: Good

Example: Bad

Example: Bad

Entity Sets Vs. 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.

Example: Good

Example: Bad

Example: Bad

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.

When To use 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.