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1–1 The E-R Model Prof. Sin-Min Lee Department of Computer Science.

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Presentation on theme: "1–1 The E-R Model Prof. Sin-Min Lee Department of Computer Science."— Presentation transcript:

1 1–1 The E-R Model Prof. Sin-Min Lee Department of Computer Science

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5 1–5 E-R Model The E-R model is not intended to be associated with any particular database model. E-R diagrams are intended to allow humans the ability to capture more of the application’s meaning.

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7 1–7 The Entity-Relationship Model (History) Developed by Peter Chen in the 1970’s Several variations have evolved All are designed towards the concise capture of the application semantics in terms appropriate for subsequent mapping to a specific database model. It is currently the most widely used.

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9 1–9 The Entity-Relationship Approach Entity: an object that exists and is distinguishable from other objects. i.e. person, place, thing, event or concept about which information(attributes) is recorded. The basic unit of the E-R model. The structure of an entity is called its schema.

10 1–10 More Terminology Object: things in the real world that can be observed and classified because they have related properties Entity: the groupings we use when we categorize the objects. Sometimes called a class.

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13 1–13 Three Basic Graphical Symbols in an ER Diagram Rectangles are used to model conceptual data units or data objects. Circles are used to model attributes. Attributes are the characteristics, components or properties of entities. Diamonds are used to model the structural associations that exist between entities. Course CID Enroll

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16 1–16 BASIC CONCEPTS There are 3 basic notions in the E-R Model: Entity Sets Relationship Sets Attributes

17 1–17 AN ENTITY An entity is a “thing” or “object” in the real world that is distinguishable from all other objects. It has an unique set of properties that may uniquely identify an entity. For example, a student entity has three attributes: name, student-id, and social-security numbers. John1222123-12-2244 STUDENT entity

18 1–18 ENTITY SETS An entity set is a set of entities that share the same properties or attributes. Entity sets do not need to be disjoint. For example, a customer can also be an employee. John1222123-12-2244 Kathy2223223-22-2245 Steve3222723-12-2244 Customer (Entity Set) JohnManager KathyTeller SteveTeller Employee (Entity Set)

19 1–19 ATTRIBUTE An entity is represented by a set of attributes. Attributes are descriptive properties possessed by each member of an entity set. Example: STUDENT ss#student-idaddressname STUDENT entity Attributes

20 1–20 ATTRIBUTE TYPES There are several different types of attributes. Simple and Composite Single-valued and Multivalued Derived Null

21 1–21 ATTRIBUTE TYPES Simple attributes are not divided into subparts. Composite attributes can be divided into subparts. Using composite attributes in a design schema is a good choice if a user will wish to refer to an entire attribute on some occasions.

22 1–22 ATTRIBUTE TYPES Example:address (Composite attribute) street citystatezip street-numberstreet-nameapartment-number Composite attribute

23 1–23 ATTRIBUTE TYPES Single-valued attributes are attributes that only have a single value for a particular entity. Multi-valued attributes refers to entities that are not singled- value and Null valued. For example, consider an employee entity set with the attribute phone-number. An employee may have zero, one, or several phone numbers, different employee may have different numbers of phones.

24 1–24 ATTRIBUTE TYPES Null attribute is used when an entity does not have a value for an attribute. Derived attributes refer to an attribute that can be derived from other related attributes or entities. For instance, suppose that Age and Date-of-birth are attributes of the CUSTOMER entity set. We can calculate Age from Date-of-birth. In this case, Age is a derived attribute.

25 1–25 RELATIONSHIP SETS A relationship is an association among several entities. A relationship set is a set of relationships of the same type. Consider the two entity sets customer and loan. We define the relationship set borrower to denote the association between customers and bank loans that the customers have. 321-12-3123JonesMainHarrison 019-28-3746SmithNorthRye 555-55-5555JacksonDupontWoodside 321-12-3123CurryNorthRye 321-12-3123AdamSpringPittsfield L-171000 L-232000 L-141500 L-19500 L-161300 customer loan

26 1–26 RECURSIVE RELATIONSHIP Recursive relationship- the same entity set participates in a relationship set more than once, in different roles. The function that an entity plays in a relationship is called that entity’s role. For example, consider an entity set employee that records information about all employees of the bank. We may have a relationship set works-for that is modeled by ordered pairs of employee entities. The first employee of a pair takes the role of worker, whereas the second takes the role of manager. employee employee-name telephone-number employee-id works-for manager worker Recursive Relationship

27 1–27 BINARY RELATIONSHIP SETS Binary relationship set - relationship that involves two identity sets. Most of the relationship sets in a database system are binary. Occasionally, however, relationship set involve more than two entity. OWNER Owns PROPERTY-FOR-RENT Binary relationship called Owns

28 1–28 TERNARY RELATIONSHIP SET Ternary relationship set - relationship that involves three identity sets DEGREE DATE PERSON earned PERSON earned DEGREE on DATE Ternary Relationship Set

29 1–29 DEGREE OF A RELATIONSHIP SET Degree of a relationship set is the number of entity sets that participate in a relationship set. For example, a binary relationship set is of degree 2; a ternary relationship set is of degree 3.

30 1–30 CONSTRAINTS An E-R enterprise scheme may define certain constraints to which the contents of a database must conform. The two of most important types of constraints are Mapping Cardinalities and Participation Constraints. Participation Constraints The participation of an entity set E in a relationship set R is said to be total, if every entity in E participates in at least one relationship in R. If only some entities in E participate in relationship R, the participation of entity set E in relationship R is said to be partial.

31 1–31 CONSTRAINTS Mapping Cardinalities or Cardinality ratios Express the number of entities to which another entity can be associated via a relationship set Are most useful in describing binary relationship sets. For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following:  One to one  One to many  Many to one  Many to many

32 1–32 MAPPING CARDINALITIES One to one An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A. A3 B2 B1 B3 A1 A2

33 1–33 MAPPING CARDINALITIES One to many An entity in A is associated with any number (zero or more) of entities in B. An entity in B, however, can be associated with at most one entity in A. A3 B3 B2 B4 A1 A2 B1

34 1–34 MAPPING CARDINALITIES Many to one An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number (zero or more) of entities in A. A3 B2 B1 B3 A1 A2 A1

35 1–35 MAPPING CARDINALITIES Many to many An entity in A is associated with any number (zero or more) of entities in B, and an entity in B is associated with any number (zero or more) of entities in A. A3 B2 B1 B3 A1 A2 A1

36 1–36 KEYS A key allows us to identify a set of attributes that suffice to distinguish entities from each other. Keys also help uniquely identify relationships, and thus distinguish relationships from each other. There are three types of keys: Super key Candidate key Primary key

37 1–37 KEY Superkey is a set of one or more attributes that, taken collectively, us to identify uniquely an entity in the entity set. For example, customer-id is a superkey. Candidate key is a minimal superkey. For example, customer-name and customer-street is sufficient to distinguish among members of the customer entity set. Then {customer-name, customer-street} is a candidate key. Primary key denotes a candidate key that is chosen by the database designer as the principal means of identifying entities within an entity set. the primary key should be chosen such that its attributes are never, or very rarely, changed. For example, Social-security numbers are guaranteed to never changed.

38 1–38 RELATIONSHIP SETS Primary key of an entity set allows us to distinguish among entities of the set. Similar mechanism is needed in order to distinguish among the various relationships of a relationship set. The structure of the primary key for the relationship set depends on the mapping cardinality of the relationship set.

39 1–39 RELATIONSHIP SETS For example, suppose that there is a relationship set Depositor, with attribute access-date, between Customer relation and Account relation. Suppose that the relationship set is many to many. Then the primary key of Depositor consists of the union of the primary keys of Customer and Account. If a customer can have only one account- that is, if the Depositor relation is many to one from Customer to Account- then the primary key of the Depositor relationship is simply the primary key of customer.

40 1–40 RELATIONSHIP SETS Customer (ss#, name) Account (account-num) Depositor (ss#, account-num, access-date) John123-12-2244 Kathy223-22-2245 Steve723-12-2244 Customer A-101 A-215 A-102 Account 24 May 2002 3 June 2002 20 June 2002 21 June 2002 10 May 2002 Depositor

41 1–41 DESIGN ISSUE Use of Entity Sets versus Attributes Consider the entity set employee with attributes employee- name and telephone-number. Treating a telephone as an attribute telephone-number implies that employees have precisely one telephone number each. Treating a telephone as an entity telephone permits employees to have several telephone number ( including zero) associated with them. However, we could instead easily define telephone-number as a multivalued attribute to allow multiple telephones per employee.

42 1–42 USE OF ENTITY SETS VERSUS ATTRIBUTES Treating a telephone as an attribute Treating a telephone as an entity 408-888-3565cell-phone 408-345-5366video-phone 408-222-3455cell-phone 714-555-6366cell-phone John408-345-5366 Kathy408-222-3455 Steve714-555-6366 EMPLOYEE (employee-name, telephone-number) John Kathy Steve Alex TELEPHONE(telephone-number, type)EMPLOYEE (employee-name)

43 1–43 ENTITY-RELATIONSHIP DIAGRAM Ellipses, which represent attributes Diamonds, which represent relationship sets Line, which link attributes to entity sets and entity sets to relationships sets Double ellipses, which represent multivalued attributes Double lines, which indicate total participation of an entity in a relationship set Double rectangles, which represent weak entity sets

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