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RDBMS ERD

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  • 1. The Entity Relationship Model
  • 2. Data Modeling Data modeling – a technique for organizing and documenting a system’s data. Sometimes called database modeling. 2 Entity relationship diagram (ERD) – a data model utilizing several notations to depict data in terms of the entities and relationships described by that data.
  • 3. Purpose of E-R Model • Facilitates database design • Express logical properties of mini-world of interest within enterprise - Universe of DiscourseDiscourse • Conceptual level model • Not limited to any particular DBMS • E-R diagrams used as design tools • A semantic model – captures meanings
  • 4. Symbols used in E-R Diagram • Entity – rectangle • Attribute – oval • Relationship – diamond • Link - line• Link - line
  • 5. Course Number Title Credit Hours ER diagram Section StudentTakes Grade Belongs Term Semester Year Id Name e-mail Number
  • 6. Entity • Object that exists and that can be distinguished from other objects • Can be person, place, event, object, concept in the real world • Can be physical object or abstraction • Entity instance is a particular person, place, etc. • Entity type is a category of entities • Entity set is a collection of entities of same type-must be well-defined • In E-R diagram, rectangle represents entity set
  • 7. Data Modeling Concepts: Entity Entity instance – a single occurrence of an entity. Student ID Last Name First Name 2144 Arnold Betty entity 8-7 2144 Arnold Betty 3122 Taylor John 3843 Simmons Lisa 9844 Macy Bill 2837 Leath Heather 2293 Wrench Tim instances
  • 8. Data Modeling Concepts: Degree Associative entity – an entity that inherits its primary key from more than one other entity (called parents). 8-8 Each part of that concatenated key points to one and only one instance of each of the connecting entities. Associative Entity
  • 9. Attributes • Defining properties or qualities of entity type • Represented by oval on E-R diagram • Domain – set of allowable values for attribute – Credit hours might be integer values between 0 and 150 – lastName might be all legal last names – a string that might also include apostrophes, blanks, hyphens, or other special characterscharacters • Attribute maps entity set to domain • May have null values for some entity instances – no mapping to domain for those instances
  • 10. Attributes (cont.) • May be multi-valued – use double oval on E-R diagram (e.g., student may have more than one email address) • May be composite – use oval for composite attribute, with ovals for components connected to it by lines address emailStudents • May be derived – use dashed oval address street city state zip Faculty age
  • 11. Keys • Superkey: attribute or set of attributes that uniquely identifies an entity (can always tell one entity instance from another) – stuId is superkey for Student entity – stuId, credits together form a superkey, because stuId is a superkey • Composite key: key with more than one attribute – courseNumber, sectionNumber, semester make up a composite key • Candidate key: superkey such that no proper subset of its attributes is also a superkey (minimal superkey –no unnecessary attributes) – Although stuId, credits is a superkey, only stuId is a candidate key – courseNumber, sectionNumber, semester is a candidate (no one attribute is a superkey) – If Student entity contains stuId and ssan as atributes, both stuId and ssan are candidate keys
  • 12. Keys (cont.) • Primary key: the candidate key actually used for identifying entities and accessing records • Alternate key: candidate key not used for primary key • Secondary key: attribute or set of attributes used for accessing records, but not necessarily unique – lastName might be used to find instances in Student, to help narrow down the resultshelp narrow down the results • Foreign key: term used in relational model (but not in the E-R model) for an attribute that is primary key of a table and is used to establish a relationship, usually with another table, where it appears as an attribute also – stuId in Enroll entity
  • 13. Relationships • Connections or interactions between entity instances • Represented by diamond on E-R diagram • Relationship type – category of relationships • Relationship set – collection of relationships of same type, consists of relationship instances – relationships that exist at a given moment • Type forms intension; set forms extension of relationship• Type forms intension; set forms extension of relationship • Relationship can have descriptive attributes • Degree of relationship – Binary – links two entity sets; set of ordered pairs – Ternary – links three entity sets; ordered triples – N-ary – links n entity sets; ordered n-tuples – Note: ternary relationships may sometimes be replaced by two binary relationships
  • 14. Replace Ternary Relationship with Two Binary Relationships Ternary Relationship Binary Relationship
  • 15. Cardinality of Relationships • Number of entity instances to which another entity can map under the relationship • One-to-one: X:Y is 1:1 is each entity in X is associated with at most one entity in Y and each entity in Y with at most one entity in X. • One-to-many: X:Y is 1:M is each entity in X can be associated• One-to-many: X:Y is 1:M is each entity in X can be associated with many entities in Y, but each entity in Y with at most one entity in X. • Many-to-many: X:Y is M:M if each entity in X can be associated with many entities in Y, and each entity in Y with many entities in X (many=more than one)
  • 16. Showing Cardinalities on ER Diagram One: 1 Many: M, n Faculty ClassFaculty- Class 1 M Chair- 11 Student ClassEnroll NM Chairperson Department Chair- Dept 11
  • 17. Chair- Showing Cardinalities on ER Diagram One: Single Arrow Many: Double Arrow Faculty ClassFaculty- Class Chairperson Department Chair- Dept Student ClassEnroll
  • 18. Faculty ClassFaculty- Class Chair- Showing Cardinalities on ER Diagram One: Single Arrow Many: No Arrow Chairperson Department Chair- Dept Student ClassEnroll
  • 19. Showing Cardinalities on ER Diagram One: No Arrow Many: Big Dot Faculty ClassFaculty- Class Chairperson Department Chair- Dept Student ClassEnroll
  • 20. Showing Cardinalities on ER Diagram One: No Arrow Many: Crow’s Feet Faculty ClassFaculty- Class Chairperson Department Chair- Dept Student ClassEnroll
  • 21. Cardinality Example
  • 22. Recursive Relationship
  • 23. Entity Sets with Two Relationships
  • 24. Existence Dependency and Weak Entities • Entity Y is existence dependent on entity X is each instance of Y must have a corresponding instance of X • Y must have total participation in its relationship with X • If Y does not have its own candidate key, Y is called a weak entity, and X is strong entity • Weak entity may have a partial key, a discriminator, that• Weak entity may have a partial key, a discriminator, that distinguishes instances of the weak entity that are related to the same strong entity • Use double rectangle for weak entity, with double diamond for relationship connecting it to its strong entity
  • 25. Data Modeling Concepts: Parent and Child Entities Parent entity - a data entity that contributes one or more attributes to another entity, called the child. In a one-to-many relationship the parent is the entity on the "one" side. 8-25 Child entity - a data entity that derives one or more attributes from another entity, called the parent. In a one-to-many relationship the child is the entity on the "many" side.
  • 26. Data Modeling Concepts: Generalization Generalization – a concept wherein the attributes that are common to several types of an entity are grouped into their own entity. Supertype – an entity whose instances store attributes 8-26 Supertype – an entity whose instances store attributes that are common to one or more entity subtypes. Subtype – an entity whose instances may inherit common attributes from its entity supertype And then add other attributes unique to the subtype.
  • 27. Entity Discovery • In interviews or JRP sessions, pay attention to key words (i.e. "we need to keep track of ..."). • In interviews or JRP sessions, ask users to identify things about which they would like to capture, 8-27 things about which they would like to capture, store, and produce information. • Study existing forms, files, and reports. • Scan use case narratives for nouns. • Some CASE tools can reverse engineer existing files and databases.
  • 28. ER Diagram Example
  • 29. Existence Dependency and Weak Entities Example Faculty facId name rank Evaluation IsRated date rater rating
  • 30. ER Diagram Example deptName Department deptCode office EmploysChairsHasMajor Offers Textbook isbn title credit Student stuId major lastName firstName lastName Faculty facId rank firstName Faculty- Class- Textbook Teaches Textbook author publisher
  • 31. E-R Diagram description • Student: stuId, lastName, firstName, major, credits – Each student has a unique id and has at most one major • Department: deptCode, deptName, office – Each department has a unique code and a unique name, and that each department has one office designated as the departmental office • Faculty: facId, lastName, firstName, rank – facId is unique and that every faculty member must belong to department. One faculty member in each department is thedepartment. One faculty member in each department is the chairperson. • Class: classNumber, sched, room – classNumber consists of deptCode, courseNumber, section • Textbook: isbn, author, title, publisher – A book can have multiple authors • Evaluation: date, rater, rating – Evaluation is a weak entity, dependent on Faculty