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1–2 1–3 1–4 1–5 Entity-Relationship (E-R) Model COURSE# CTITILE PROF SCHED ROOM COURSE ENROLL GRADE STUDENT STUID SNAME MAJOR CREDITS 1–6 Class Hierarchies name lot ssn * Sometimes it’s natural to classify the entities in an entity set into subclass, then we will use Class Hierarchies. Employees Hourly wage Hours worked Hourly Emps ISA contractid Contract Emps 1–7 * A relationship set is an association between Aggregation name entity sets. ssn Sometimes we have to model a relationship between a collection of entities and relationships, then we need to use aggregation. lot Employees monitors until Started-on dname since pid pbudget Projects did Sponsors budget Department 1–8 Conceptual design using the ER model • Should a concept be modeled as an entity or an attribute ? • Should a concept be modeled as an entity or a relationship ? • What are the relationships ? Should we use binary or ternary relationships ? • Should we use aggregation ? 1–9 UNARY RELATIONSHIP • Unary relationship : is an associate of 1 entity set. * Note : an entity set is a group of related entities. Student Dr. Lee * Entity set = * Relation = * Attribute = 1–10 BINARY RELATIONSHIP • Binary relationship : is an associate of 2 entities sets . Employees ssn name Department Works_In lot did dname budget * Entity set = * Relation = * Attribute = 1–11 TERNARY RELATIONSHIP Ternary relationship : is an associate of 3 entities sets . Employees ssn name Department Works_In lot add did Locations dname budget cap * Entity set = * Relation = * Attribute = 1–12 QUARTERY RELATIONSHIP Quartery relationship : is an associate of 4 entities sets. Product Employees * Entity set = Works_In Department Locations * Relation = * Attribute = 1–13 1–14 Additional features of the ER model • In the ER model allow us to draw important distinctions about the data. • One of the features of the ER model is : Key Constraints Note : A key constraints is also known as Primary key. A primary key is a candidate key selected to uniquely identify all other attribute values in any given row, can not contain null entries. 1–15 An example of Key Constraints on Manage name ssn Since did lot Employees dname Manages budget Department ssn = Social Security Number. did = Department id. 1–16 1–17 Weak Entities name ssn cost Pname --------- lot Employees Policy age Department A weak entity : is an entity of which is existence depends on other entities. 1–18 1–19 1–20 Entity versus Attribute from name ssn dname did lot Employees to Works-In2 budget Department * Entity set = * Relation = * Attribute = 1–21 Entity versus Relationship dname name budget did ssn lot Employees Manages Department since apptnum Mgr-Appts dbudget 1–22 Binary versus Ternary Relationship name ssn Pname --------- lot Employees Covers age Dependents Policies policyid cost 1–23 Using a Ternary Relationship instead of Started-on dname Aggregation pid did pbudget Projects Sponsors budget Departments Employees lot ssn name 1–24 ONE TO MANY RELATIONSHIP customer-name customer-street loan-number customer-city customer-id customer borrower amount loan 1–25 MANY TO ONE RELATIONSHIP customer-name customer-street loan-number customer-city customer-id customer borrower amount loan 1–26 ONE TO ONE RELATIONSHIP customer-name customer-street loan-number customer-city customer-id customer borrower amount loan 1–27 E-R Diagram with Composite, Multivalued, and Derived Attributes street-name middle-name Street-number apartment-number last-name first-name street name city customer-id address Customer state age zip-code date-of-birth phone-number 1–28 WEAK ENTITY SETS An entity set may not have sufficient attributes to form a primary key. Such an entity set is termed a weak entity set. An entity set that has a primary key is termed a strong entity set. For a weak entity set to be meaningful, it must be associated with another entity set, called the identifying or owner entity set. The relationship associating the weak entity set with the identifying entity set is called the identifying relationship. The identifying relationship is many to one from the weak entity set to the identifying entity set, and the participation of the weak entity set in the relationship is total. 1–29 DISCRIMINATOR The discriminator of a weak entity set is a set of attributes that allows this distinction to be made. For example, the discriminator of a weak entity set payment is the attribute payment-number, since, for each loan a payment number uniquely identifies one single payment for that loan. The discriminator of a weak entity set is also called the partial key of the entity set. Note: although each payment entity is distinct, payments for different loans may share the same payment-number. Thus, payment entity set does not have a primary key; it is a weak entity set. The primary key of a weak entity set is formed by the primary key of the identifying entity set, plus the weak entity set’s discriminator. 1–30 E-R DIAGRAM WITH A WEAK ENTITY SET In a E-R diagrams, a doubly outlined box indicates a weak entity set, and a doubly outlined diamond indicates the corresponding identifying relationship. We underline the discriminator of a weak entity set with a dashed line. payment-date amount loan-number payment-number loan loan-payment payment-amount payment E-R diagram with a weak entity set 1–31 1–32 1–33 1–34 1–35 Entity/Relationship Model Diagrams to represent designs. • Entity like object, = “thing.” • Entity set like class = set of “similar” entities/objects. • Attribute = property of entities in an entity set, similar to fields of a struct. • In diagrams, entity set rectangle; attribute oval. ID name Students phone height 1–36 Relationships • Connect two or more entity sets. • Represented by diamonds. Students Taking Courses 1–37 Relationship Set Think of the “value” of a relationship set as a table. • One column for each of the connected entity sets. • One row for each list of entities, one from each set, that are connected by the relationship. Students Sally Sally Joe … Courses CS180 CS111 CS180 … 1–38 Multiway Relationships Usually binary relationships (connecting two E.S.) suffice. • However, there are some cases where three or more E.S. must be connected by one relationship. • Example: relationship among students, courses, TA's (and graders). Students Taking Courses Possibly, this E/R diagram is OK: Assisting TA/Graders 1–39 • Works in CS180, because each TA (or grader) is a TA of all students. Connection student-TA is only via the course. • But what if students were divided into sections, each headed by a TA? Then, a student in CS180 would be related to only one of the TA's for CS180. Which one? • Need a 3-way relationship to tell. 1–40 Courses Students Enrolls TAs Students Ann Sue Bob … Courses CS180 CS180 CS180 … TAs Jan Pat Jan … 1–41 Beers-Bars-Drinkers Example • Our running example for the course. name addr license Serves Bars Frequents Beers Likes Drinkers name manf name addr 1–42 Multiplicity of Relationships Many-many Many-one One-one Representation of Many-One • E/R: arrow pointing to “one.” Rounded arrow = “exactly one.” 1–43 Example: Drinkers Have Favorite Beers name Serves addr Bars license Frequents Likes Beers name manf Drinkers Favorite name addr 1–44 One-One Relationships Put arrows in both directions. Manfs Bestseller Beers Design Issue: Is the rounded arrow justified? Design Issue: Here, manufacturer is an E.S. In earlier diagrams it is an attribute. Which is right? 1–45 Attributes on Relationships price Bars Sells Beers • Shorthand for 3-way relationship: price Prices Bars Sells Beers 1–46 • A true 3-way relationship. Price depends jointly on beer and bar. • Notice arrow convention for multiway relationships: “all other E.S. determine one of these.” Not sufficiently general to express any possibility. However, if price, say, depended only on the beer, then we could use two 2-way relationships: price-beer and beer-bar. Or better: just make price an attribute of beer. 1–47 Converting Multiway to 2-Way • Baroque in E/R, but necessary in certain “object-oriented” models. • Create a new connecting E.S. to represent rows of a relationship set. E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship. • Many-one relationships from the connecting E.S. to the others. BBP TheBar TheBeer ThePrice Bars Beers Price 1–48 Roles Sometimes an E.S. participates more than once in a relationship. • Label edges with roles to distinguish. Married husband wife Husband d1 d3 … Wife d2 d4 … Drinkers 1–49 Buddies 1 2 Drinkers Buddy1 d1 d1 d2 d2 … Buddy2 d2 d3 d1 d4 … • Notice Buddies is symmetric, Married not. No way to say “symmetric” in E/R. Design Question Should we replace husband and wife by one relationship spouse? 1–50 More Design Issues 1. Subclasses. 2. Keys. 3. Weak entity sets. (Next class.) 1–51 Subclasses Subclass = special case = fewer entities = more properties. • Example: Ales are a kind of beer. In addition to the properties (= attributes and relationships) of beers, there is a “color” attribute for ales. 1–52 E/R Subclasses • Assume subclasses form a tree (no multiple inheritance). • isa triangles indicate the subclass relation. name Beers manf isa color Ales 1–53 Different Subclass Viewpoints 1. E/R viewpoint: An entity has a component in each entity set to which it logically belongs. Its properties are the union of the properties of these E.S. 2. Contrasts with object-oriented viewpoint: An object (entity) belongs to exactly one class. It inherits properties of its superclasses. name Beers isa color manf Pete’s Ale Ales 1–54 Multiple Inheritance Theoretically, an E.S. could be a subclass of several other entity sets. name manf name manf Beers Wines isa isa Grape Beers 1–55 Problems How should conflicts be resolved? • Example: manf means vintner for wines, bottler for beers. What does manf mean for “grape beers”? • Need ad-hoc notation to resolve meanings. • In practice, we shall assume a tree of entity sets connected by isa, with all “isas” pointing from child to parent. 1–56 Keys A key is a set of attributes whose values can belong to at most one entity. • In E/R model, every E.S. must have a key. It could have more than one key, but one set of attributes is the “designated” key. • In E/R diagrams, you should underline all attributes of the designated key. 1–57 Example • Suppose name is key for Beers. name Beers manf isa color Ales • Beer name is also key for ales. In general, key at root is key for all. 1–58 Example: A Multiattribute Key number dept Courses hours room • Possibly, the combination of hours + room also forms a key, but we have not designated it as such. 1–59
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