Presentation on theme: "1–1 The E-R Model Prof. Sin-Min Lee Department of Computer Science."— Presentation transcript:
1–1 The E-R Model Prof. Sin-Min Lee Department of Computer Science
1–6 Entity-Relationship (E-R) Model COURSE # CTITILEPROFSCHED ROOM COURSE ENROLL STUDENT STUID SNAME MAJOR CREDITS GRADE
1–7 Class Hierarchies Employees ssn name lot ISA Hourly Emps Contract Emps Hourly wage Hours worked contractid * Sometimes it’s natural to classify the entities in an entity set into subclass, then we will use Class Hierarchies.
1–8 Aggregation name ssnlot Employees monitors until since Sponsors DepartmentProjects pid Started-on pbudget dname didbudget * A relationship set is an association between entity sets. Sometimes we have to model a relationship between a collection of entities and relationships, then we need to use aggregation.
1–9 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 ?
1–10 UNARY RELATIONSHIP Unary relationship : is an associate of 1 entity set. Note * Note : an entity set is a group of related entities. Student * Entity set = * Relation = Dr. Lee * Attribute =
1–11 BINARY RELATIONSHIP Binary relationship : is an associate of 2 entities sets. Employees Works_In Department * Entity set = * Relation = * Attribute = ssnnamelot did budget dname
1–12 TERNARY RELATIONSHIP Works_In EmployeesDepartment Locations Ternary relationship : is an associate of 3 entities sets. * Entity set = * Relation = ssn name lotdid dname budget capadd * Attribute =
1–13 QUARTERY RELATIONSHIP Employees Works_In Locations Department Product Quartery relationship : is an associate of 4 entities sets. * Entity set = * Relation = * Attribute =
1–15 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 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.
1–16 An example of Key Constraints on Manage Manages Department Employees ssn name lot Sincedname did budget ssn = Social Security Number. did = Department id.
1–18 Weak Entities A weak entity : is an entity of which is existence depends on other entities. Policy Department Employees ssn name lot cost Pname age
1–21 Entity versus Attribute Works-In2 Department Employees ssn name lot from dname did budget to * Entity set = * Relation = * Attribute =
1–22 Entity versus Relationship Manages Department Employees ssn name apptnum did budget dname Mgr-Appts lot since dbudget
1–23 Binary versus Ternary Relationship Covers Dependents Employees ssn name Pname age Policies lot policyidcost
1–24 Using a Ternary Relationship instead of Aggregation Sponsors Departments Projects pid Started-on did budget pbudget dname Employees name ssn lot
1–25 ONE TO MANY RELATIONSHIP customer customer-name customer-city customer-id borrower customer-street loan loan-number amount
1–26 MANY TO ONE RELATIONSHIP customer customer-name customer-city customer-id borrower customer-street loan loan-number amount
1–27 ONE TO ONE RELATIONSHIP customer customer-name customer-city customer-id borrower customer-street loan loan-number amount
1–28 E-R Diagram with Composite, Multivalued, and Derived Attributes Customer date-of-birth phone-number name customer-id first-name age last-name middle-name zip-code state city address street Street-numberapartment-number street-name
1–29 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.
1–30 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.
1–31 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. loan E-R diagram with a weak entity set loan-payment payment payment-date payment-amount payment-number amount loan-number
1–36 Students 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 namephone height
1–37 Relationships Connect two or more entity sets. Represented by diamonds. StudentsCourses Taking
1–38 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. StudentsCourses SallyCS180 SallyCS111 JoeCS180 ……
1–39 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). Possibly, this E/R diagram is OK: StudentsCourses Taking Assisting TA/Graders
1–40 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? u 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.
1–41 StudentsCoursesTAs AnnCS180Jan SueCS180Pat BobCS180Jan ……… Students Courses TAs Enrolls
1–42 Beers-Bars-Drinkers Example Our running example for the course. nameaddrlicense namemanfnameaddr BeersDrinkers Bars Serves Frequents Likes
1–43 Multiplicity of Relationships Representation of Many-One E/R: arrow pointing to “one.” u Rounded arrow = “exactly one.” Many-many Many-oneOne-one
1–45 One-One Relationships Put arrows in both directions. 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? ManfsBeers Best- seller
1–46 Attributes on Relationships Shorthand for 3-way relationship: BarsBeers Sells price BarsBeers Sells price Prices
1–47 A true 3-way relationship. u Price depends jointly on beer and bar. Notice arrow convention for multiway relationships: “all other E.S. determine one of these.” u Not sufficiently general to express any possibility. u However, if price, say, depended only on the beer, then we could use two 2-way relationships: price-beer and beer-bar. u Or better: just make price an attribute of beer.
1–48 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. u E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship. Many-one relationships from the connecting E.S. to the others. BarsBeers The- Bar Price The- Beer The- Price BBP
1–49 Roles Sometimes an E.S. participates more than once in a relationship. Label edges with roles to distinguish. HusbandWife d 1 d 2 d 3 d 4… Drinkers Married husbandwife
1–50 Notice Buddies is symmetric, Married not. u No way to say “symmetric” in E/R. Design Question Should we replace husband and wife by one relationship spouse? Buddy1Buddy2 d 1 d 2 d 1 d 3 d 2 d 1 d 2 d 4… Drinkers Buddies 12
1–52 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.
1–53 E/R Subclasses Assume subclasses form a tree (no multiple inheritance). isa triangles indicate the subclass relation. namemanf Beers Ales color isa
1–54 Different Subclass Viewpoints 1. E/R viewpoint: An entity has a component in each entity set to which it logically belongs. u 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. u It inherits properties of its superclasses. namemanf Beers Ales color isa Pete’s Ale
1–55 Multiple Inheritance Theoretically, an E.S. could be a subclass of several other entity sets. namemanf Beers namemanf Wines Grape Beers isa
1–56 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.
1–57 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. u 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.
1–58 Example Suppose name is key for Beers. Beer name is also key for ales. u In general, key at root is key for all. namemanf Beers Ales color isa
1–59 Example: A Multiattribute Key Possibly, the combination of hours + room also forms a key, but we have not designated it as such. deptroom Courses numberhours