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Chapter 4 Entity Relationship (E-R) Modeling

Chapter 4 Entity Relationship (E-R) Modeling. Database Systems: Design, Implementation, and Management Peter Rob & Carlos Coronel. In this chapter, you will learn:.

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Chapter 4 Entity Relationship (E-R) Modeling

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  1. Chapter 4Entity Relationship (E-R) Modeling Database Systems: Design, Implementation, and Management Peter Rob & Carlos Coronel

  2. In this chapter, you will learn: • How relationships between entities are defined and refined, and how such relationships are incorporated into the database design process • How ERD components affect database design and implementation • How to interpret the modeling symbols for the four most popular ER modeling tools • That real-world database design often requires that you reconcile conflicting goals

  3. The Entity Relationship (E-R) Model • ER model forms the basis of an ER diagram • ERD represents the conceptual database as viewed by end user • Main Components • Entities • In E-R models an entity refers to the entity set. • An entity is represented by a rectangle containing the entity’s name. • Attributes • Attributes are represented by ovals and are connected to the entity with a line. • Each oval contains the name of the attribute it represents. • Attributes have a domain -- the attribute’s set of possible values. • Attributes may share a domain. • Relationships

  4. The Attributes of the STUDENT Entity

  5. Primary Keys • Underlined in the ER diagram • Key attributes are also underlined in frequently used table structure shorthand • Ideally composed of only a single attribute • Possible to use a composite key: • Primary key composed of more than one attribute

  6. The CLASS Table (Entity) Components and Contents

  7. The Entity Relationship (E-R) Model • Classes of Attributes • A simple attributecannot be subdivided. • Examples: Age, Sex, and Marital status • A composite attribute can be further subdivided to yield additional attributes. • Examples: • ADDRESS Street, City, State, Zip • PHONE NUMBER  Area code, Exchange number

  8. The Entity Relationship (E-R) Model • Classes of Attributes • A single-valued attribute can have only a single value. • Examples: • A person can have only one social security number. • A manufactured part can have only one serial number. • Multivalued attributes can have many values. • Examples: • A person may have several college degrees. • A household may have several phones with different numbers • Multivalued attributes are shown by a double line connecting to the entity.

  9. Multivalued attributes in an Entity

  10. The Entity Relationship (E-R) Model • Multivalued Attribute in Relational DBMS • The relational DBMS cannot implement multivalued attributes. • Possible courses of action for the designer • Within the original entity, create several new attributes, one for each of the original multivalued attribute’s components ( Figure 4.4). • Create a new entity composed of the original multivalued attribute’s components ( Figure 4.5).

  11. Splitting the Multivalued Attribute into New Attributes

  12. A New Entity SetComposed of Multivalued Attribute’s Components

  13. The Entity Relationship (E-R) Model • A derived attribute • may be calculated (derived) from other attributes • Need not be physically stored within the database • Can be derived by using an algorithm • Example: AGE can be derived from the data of birth and the current date.

  14. Depiction of a Derived Attribute

  15. The Entity Relationship (E-R) Model • Relationships • A relationship is an association between entities. • Relationships are represented by diamond-shaped symbols.

  16. The Entity Relationship (E-R) Model • Connectivity • The term connectivity is used to describe the relationship classification (e.g., one-to-one, one-to-many, and many-to-many).

  17. The Entity Relationship (E-R) Model • Cardinality • Cardinality expresses the specific number of entity occurrences associated with one occurrence of the related entity. • The minimum and maximum number of entity occurrences

  18. Connectivity and Cardinality in an ERD

  19. Relationship Strength • Existence Dependent • If an entity’s existencedepends onthe existence of one or more other entities, it is said to be existence-dependent. • CLASS is existence-dependent on COURSE (parent entity) • EMPLOYEE claims DEPENDENT—DEPENDENT is existence-dependent on EMPLOYEE • Existence independent • Entity can exist apart from one or more related entities • Example: • some of parts are produced “in-house” and other parts are bought from vendors.At least some of the parts are not supplied by a vender. • PART is existence-independent from VENDOR

  20. Relationship Strength • Weak (non-identifying) relationship • One entity is notexistence-independent on another entity • PK of related entitydoesn’t contain PK component of parent entity • COURSE( CRS_CODE, …) • CLASS( CLASS_CODE, …) • Strong (identifying)relationship • One entity is existence-dependent on another • PK of related entity contains PK component of parent entity • COURSE( CRS_CODE, …) • CLASS( CRS_CODE, CLASS_SECTION, …)

  21. A Weak Relationship Between COURSE and CLASS

  22. A Strong (Identifying) Relationship Between COURSE and CLASS

  23. The Entity Relationship (E-R) Model • Relationship Participation • Optional • The participation is optional if one entity occurrence does not require a corresponding entity occurrence in a particular relationship. • An optional entity is shown by a small circle on the side of the optional entity. • Mandatory • Entity occurrence requires corresponding occurrence in related entity • If no optionality symbol is shown on ERD, it is mandatory

  24. CLASS is Optional to COURSE • COURSE is Mandatory to CLASS • COURSE and CLASS in a Mandatory Relationship

  25. Relationship Strength and Weak Entities • Weak Entities • A weak entity is an entity that • Is existence-dependent and • Has a primary key that is partially or totallyderived from the parent entity in the relationship. • The existence of a weak entity is indicated by a double rectangle. • The weak entity inheritsall or part of its primary key from its strong counterpart.

  26. A Weak Entity in an ERD • EMPLOYEE( EMP_NUM, EMP_LNAME, EMP_FNAME, EMP_INITIAL, EMP_DOB ) • DEPENDENT(EMP_NUM, DEP_NUM, DEP_FNAME, DEP_DOB )Primary Key DEP_NUM

  27. Weak entity in a Strong Relationship Between DEPENDENT and EMPLOYEE ( EMP_NUM ) ( EMP_NUM + DEP_NUM )

  28. Weak entity in a Strong Relationship • Weakrelationship • One entity is not existence-independent on another • PK of related entity doesn’t contain PK component of parent entity • COURSE( CRS_CODE, …) • CLASS( CLASS_CODE, …) • Strongrelationship • One entity is existence-dependent on another • PK of related entity contains PK component of parent entity • COURSE( CRS_CODE, …) • CLASS( CRS_CODE, CLASS_SECTION, …) • In any case, CLASS is always existence-dependent on COURSE, whether or not it is defined to be weak. Not Weak entity

  29. Relationship Degree • A relationship’s degree indicates the number of associated entities or participants. • A unary relationship exists when an association is maintained within a single entity. • A binary relationship exists when two entities are associated. • A ternary relationship exists when three entities are associated.

  30. The Implementation of a Ternary Relationship • 若 FUND(FUND_ID,FUND_NAME)，透過 CFR仍能將所有必要的關係聯繫起來，但 FUND(FUND_ID,FUND_NAME,CONTRIB_ID,FUND_AMOUNT)亦有意義，例如：F2 接受 C3 $10,000 的捐款，還未撥給任何 RECIPIENT Researchers

  31. The Entity Relationship (E-R) Model • Recursive relationship • A recursive relationship is one in which a relationship can exist between occurrences of the same entity set. • A recursive entity is found within a unary relationship.

  32. 1:1 Recursive relationship • EMPLOYEE is married to EMPLOYEE

  33. 1:M Recursive relationshipPART contains PART • each part is used to create only onerotor assembly • C130 = 4× AA21-6 + 2× AB-121 + …

  34. M:N Recursive relationship • PART Contains PART • A part_ can be used to create several different kinds of other parts • A part_ is itself composed of many parts. PART PART PART contains contains PART PART COMPONENT

  35. M:N Recursive relationship • COURSE Requires COURSE

  36. Implementation of the 1:M “EMPLOYEE Manages EMPLOYEE” Recursive Relationship

  37. The Entity Relationship (E-R) Model • Composite Entities • A composite entityis composed of the primary keys of each of the entities to be connected. • The composite entity serves as a bridge between the related entities. • The composite entity may contain additional attributes.

  38. The Entity Relationship (E-R) Model • Composite Entities

  39. Converting the M:N Relationship Into Two 1:M Relationships

  40. At the start of registration • A class may exist even though it contains no students at all • A student has not yet signed up for any classes.

  41. A Composite Entity in the ERD

  42. The Entity Relationship (E-R) Model • Entity Supertypes and Subtypes • Describing the different types of employees within a single entity would be awkward at best. • Example : Aviation business ( Figure 4.27)the special pilot characteristics (EMP_LICENCE, EMP_RATING, EMP_MED_TYPE) would cause a large number of nulls for other employees who are not pilots.

  43. Nulls Created by Unique Attributes

  44. The Entity Relationship (E-R) Model • Generalization hierarchy • Depicts relationships between higher-level supertype and lower-level subtype entities. • Supertype contains the shared attributes • Subtype contains the unique attributes. • A subtype entityinheritsits attributes and its relationshipsfrom the supertype entity.

  45. A Generalization Hierarchy • Disjoint relationships are indicated by G

  46. The Entity Relationship (E-R) Model • Disjoint Supertypes • Also known as non-overlapping subtypes • Subtypes that contain a subset of the supertype entity set • Each entity instance (row) of the supertype can appear in only one of the disjoint subtypes. • The supertype and its subtype(s) maintain a 1:1 relationship.

  47. The EMPLOYEE/PILOT Supertype/Subtype Relationship

  48. A Generalization Hierarchy with Overlapping Subtypes • Overlapping relationships are indicated by Gs

  49. A Comparison of ER Modeling Symbols

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