COMP 5138 Relational Database Management Systems
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1 COP 5138 Relational Database anagement Systems Dr. Uwe Roehm Lecture 2 Conceptual Database Design Review of Last Class Course Overview General organization Unit of study outline Assessment package Introduction to Database Systems File systems vs. DBS Levels of abstraction Data independence Stakeholders 2-2
2 Today s Agenda Introduction Entity Relationship odel Enhanced Entity Relationship odel Unified odel Language 2-3 Database Design Sequence Requirements Analysis Conceptual Design Logical Design Understand what data is to be stored what applications must be built what operations are most frequent Develop high-level description of the data which closely matches how users think of the data Convert conceptual design into a logical schema Physical Design Convert logical schema into a physical schema 2-4
3 Conceptual Data odel Conceptual data model A technique for understanding and capturing business information requirements graphically First phase of an effective database design Works as a communication vehicle Facilitate the planning, operation, and maintenance of various data resource 2-5 Types of Conceptual Data odels Entity Relationship odel (ER) Object-oriented Data odels Unified odelling Language (UL) OT, Booch, Etc. Object Role odelling (OR) Semantic Object odel (SO) Semantic Data odels (SD) KL-ONE etc. 2-6
4 Today s Agenda Introduction Entity Relationship odel Enhanced Entity Relationship odel Unified odel Language 2-7 Entity Relationship Diagram ERD A data modeling tool that depicts the associations among different categories of data within a business or information system. It does not imply how data is implemented, created, modified, used, or deleted. An ERD is database independent. 2-8
5 Entity-Relationship Diagram ERD First designed by Peter Chen in Other variations have since appeared. Used to interpret, specify, and document database systems. Graphical representation of what data needs to be contained in the system. 2-9 Entity-Relationship Diagram ERD Components Entities Attributes Relationships Entity
6 Entity An entity is a person, place, object, event, or concept about which you want to gather and store data. An entity type (or entity set) is a collection of entities that share common properties or characteristics. Example: STUDENT, PROFESSOR, SUBJECT Rectangle represent entity type STUDENT PROFESSOR SUBJECT An entity instance is a single occurrence of an entity type. Example: Jane Smith, a Geo etro 2-11 Attribute Once you have identified the entities you are interested in, determine which characteristics or properties about them to gather and store. These characteristics are called attributes. A named characteristics of an entity Ellipses represent attributes The attribute(s) which uniquely identifies each entity is the identifier or primary key Underline indicates primary key attributes STUDENT ID NAE ADDRESS STUDENT 2-12
7 Relationship Entities are interconnected by relationships. A named association between entities. Diamonds represent relationship sets Lines link attributes to entity sets and entity sets to relationship sets. In data modeling, you identify only those relationships among entities you would like to keep track of. PROFESSOR teaches SUBJECT 2-13 Example 2-14
8 ore on Entity When we say entity in subsequent discussions, we mean entity types, not entity instances. An Entity ust Have a name [Singular, Noun] Have an identifier Have descriptive information, other than its ID How to find an Entity? (***) Find data items with names ending with number, no, num, #, or id such as customer#, product-num, student-id, or patient-no. Take the postfix (# or id) off, then you probably found an entity Take the names of the frequently used management reports, menus, and I/O screens. Some of these will show entities. Customer, Product, Stock, etc Take the names of the files and data tables frequently accessed by the firm s main transaction programs ore on Attributes Attribute domain: a set of values from which an attribute can take a value Example: age ranges from 0 to 200 An Attribute ust: Have a unique name (singular, noun) Belong to a single entity Type of an attribute Simple (e.g., address) and composite ( suburb+ state + zip code)attributes. Single-valued and multi-valued attributes E.g. multivalued attribute: phone-numbers Double ellipses represent multivalued attributes Derived attributes Can be computed from other attributes E.g. age, given date of birth Dashed ellipses denote derived attributes Phone-number Age 2-16
9 Key Attributes A key is a minimal set of attributes whose values uniquely identify an entity in the set A candidate key is an attribute that can be used as an identifier Customer-id is candidate key of customer account-number is candidate key of account (with in one bank) (account-number & BSB) is candidate key for Australian banks Although several candidate keys may exist, one of the candidate keys is selected to be the identifier or primary key 2-17 Primary Key Primary Key: ust be unique across all instances ust not change over life of instance e.g. Tax File Number, Employee Number ust always have a valid value, not null Preferably use serial number or arbitrary ID Avoid using intelligent keys A combination of keys can be used as an identifier Example: SUBJECT Subject area SUBJECT Course # Course title 2-18
10 ore on Relationships Relationships are named with present verb phrases Example, CUSTOER places ORDER: COURSES is taught by LECTURER Naming Direction Left! Right Top! Down EPLOYEE works-in DEPT has VECHILE 2-19 Relationship Degree Degree of a Relationship # of entity types involved Unary Relationship (Recursive) Binary Relationship (80%) Ternary Relationship (Simultaneous)
11 Relationship Roles Role: Entity types of a relationship need not be distinct The labels manager and worker are called roles; they specify how employee entities interact via the works-for relationship set. Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship manager EPLOYEE Works-for worker Relationship Cardinality Cardinality of a Relationship # of entity instances from each entity type involved in a particular relationship one-to-one HUSBAND is married to 1:1 WIFE one-to-many OVIE is stocked as 1:N VIDEO TAPE many-to-many STUDENT enrolls :N COURSES
12 Relationship Cardinality: Notations Various way of showing cardinality in an ERD: one-to-many FACULTY teaches SUBJECT teaches one-to-many FACULTY 1: SUBJECT 1 one-to-many FACULTY teaches SUBJECT one-to-many FACULTY teaches SUBJECT teaches one-to-many FACULTY SUBJECT one-to-many FACULTY teaches SUBJECT Example: Relationship Example CHOI Smith COP5138 COP5415 COP5206 Joe Andrew Frank Tracey COP5413 Linda Doris teaches is enrolled by FACULTY COURSE STUDENT
13 Associative Entity Associative Entity (Entitized Relationship) When :N relationship needs to be described with its own attributes, make it an entitized relationship and treat it like a regular entity Student_Name Student # Student_Add STUDENT takes COURSE Course_Char Course # Course_Title requires Appropriate place for grade??? Associative Entity (cont d) Associative Entity (conti.) IF STUDENT entity, STUDENT COURSE Student # Student Name Student_Add Grade Course # Course_Title Course_Char Andy allet st. A COP 5138 DBS Practical Jack George st. F COP 5347 E-commerce Theory IF COURSE entity Student # STUDENT Student Name Student_Add COURSE Course # Course_Title Course_Char Grade Andy allet st. COP 5138 DBS Practical A Jack George st. COP 5347 E-commerce Theory F
14 Example: Associative Entity Associative Entity (conti.) Appropriate place for grade!!! Course_Char Student_Name Student # STUDENT takes COURSE Course # Course_Title Student_Add Grade requires Student # Course # Grade COP 5138 A COP 5347 F COP 5138 F COP 5347 A Relationship: Cardinality vs. Ordinality Cardinality and Ordinality Cardinality m:n how many occurrences of the relationship? specifies a maximum number of instances associated with the other Ordinality (Optionality) :n is the relationship mandatory or optional? specifies a minimum number of instances associated with the other EPLOYEE 1:1 manages 0:N DEPT ordinality cardinality
15 Cardinality vs. Ordinality (cont d) Cardinality and Ordinality (conti.) Alternative (and commonly used) method for drawing ordinality & cardinality: has CLUB STUDENT 0: 1:N Cardinality: many-to-many Ordinality: club must have at least one student in it student does not have to belong to a club Participation Optional & andatory Participation EPLOYEE 1:1 manages 0:N DEPT andatory Participation: Each department must have exactly one manager andatory Participation: Each employee can manage several departments, but doesn t have to. 2-30
16 Cardinality/Ordinality Notations Cardinality & Ordinality Summary A B One A is always associated with exactly one B. A B One A is associated with one or more Bs. A B One A is associated with zero or one B. A B One A is associated with zero or more Bs Ternary Relationships Ternary Relationship works for DEPT EPLOYEE is connected with works on PROJECT DEPT EPLOYEE is associated with PROJECT POOR DESIGN! BETTER DESIGN! 2-32
17 Ternary Relationships Ternary Relationship A ternary relationship is not always the same as three binary relationships 2-33 Ternary Relationships (cont d) Ternary Relationship Rule 1: All binary relationships among the three entities should be :N. Otherwise, do not create the 3ry Rule 2: IF all three :N relationships are independent of each other, do not create the 3ry EPLOYEE works-on SKILL 1 PROJECT STUDENT takes SUBJECT BRANCH sales PRODUCT PROFESSOR 1 COPANY 2-34
18 Example: Ternary Relationships Ternary Relationship (conti.) Benefits of a correct 3ry relationship EPLOYEE works-on SKILL 1 PROJECT E 1 E-S 1 S E 1 E-P S P 1 P 1 E P-S 1 S P Weak Entity An entity type that does not have a primary key is referred to as a weak entity type The existence of a weak entity type depends on the existence of a identifying entity type it must relate to the identifying entity type via a total, one-to-many relationship set from the identifying to the weak entity type Identifying relationship depicted using a double diamond The discriminator (or partial key) of a weak entity type is the set of attributes that distinguishes among all the entities of a weak entity type. The primary key of a weak entity type is formed by the primary key of the strong entity type on which the weak entity type is existence dependent, plus the weak entity type s discriminator. 2-36
19 Weak Entity (cont d) We depict a weak entity type by double rectangles. We underline the discriminator of a weak entity type with a dashed line. Dep-name discriminator of the Dependent entity type Primary key for Dependent (Employee-ID, Dep-name) Employee_Name Dep_Name Employee # Employee_Add EPLOYEE 1 has DEPENDENT Dept_Age Dept_Relation 2-37 ERD Symbols Entity Relationship Associative Entity Attribute Primary Key Attribute (Identifier) ultivalued Attribute Weak Entity Connection Derived Attribute 2-38
20 ERD Rules Every entity, relationship, and attribute should be named. Each ERD must have at least one entity. Each entity may appear only once in an ERD. Each ERD must have at least one relationship. Each entity must be connected to a relationship. Each attribute must be connected to an entity or a relationship Each relationship must be connected to either at least 2 entities (binary or ternary) or to one entity twice (unary) Today s Agenda Introduction Entity Relationship odel Enhanced Entity Relationship odel Unified odel Language 2-40
21 Concepts of EER ER model in its original form did not support SPECIALIZATION/ GENERALIZATION ABSTRACTIONS Enhanced ER model Includes all modeling concepts of basic ER Additional concepts: subclasses/superclasses, specialization/generalization, categories, attribute inheritance The resulting model is called the enhanced-er or Extended ER (E2R or EER) model It is used to model applications more completely and accurately if needed It includes some object-oriented concepts, such as inheritance 2-41 Subclass and Superclass An entity type may have additional meaningful subgroupings of its entities These are called superclass/subclass relationships These are also called IS-A relationships (SECRETARY IS- A EPLOYEE, TECHNICIAN IS-A EPLOYEE, ) Superclass EPLOYEE IS-A HOURLY EPLOYEE SALARIED EPLOYEE COISSION EPLOYEE Subclasses 2-42
22 Subclass and Superclass (conti.) An entity that is member of a subclass represents the same real-world entity as some member of the superclass The subclass member is the same entity in a distinct specific role An entity cannot exist in the database merely by being a member of a subclass; it must also be a member of the superclass A member of the superclass can be optionally included as a member of any number of its subclasses Example: A salaried employee who is also an engineer belongs to the two subclasses ENGINEER and SALARIED_EPLOYEE It is not necessary that every entity in a superclass be a member of some subclass Do not create Sub/Super hierarchy unless there exist at least one specific attribute or relationship for each subclass entity 2-43 Attribute Inheritance An entity that is member of a subclass inherits all attributes of the entity as a member of the superclass It also inherits all relationships 2-44
23 Specialization Is the process of defining a set of subclasses of a superclass The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EPLOYEE based upon job type. ay have several specializations of the same superclass Example: Another specialization of EPLOYEE based in method of pay is {SALARIED_EPLOYEE, HOURLY_EPLOYEE}. Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams Attributes of a subclass are called specific attributes. TypingSpeed of SECRETARY The subclass can participate in specific relationship types. BELONGS_TO of HOURLY_EPLOYEE 2-45 Generalization The reverse of the specialization process Several classes with common features are generalized into a superclass; original classes become its subclasses CAR, TRUCK generalized into VEHICLE; both CAR, TRUCK become subclasses of the superclass VEHICLE. We can view {CAR, TRUCK} as a specialization of VEHICLE Alternatively, we can view VEHICLE as a generalization of CAR and TRUCK 2-46
24 Subclass/Superclass Refining disjointness and completeness constraints for generalisation/specialistaion hierarchies: Disjoint an entity can belong to only one lower-level entity type Noted in E-R diagram by writing disjoint next to the ISA triangle Example: (HUAN: AN and WOAN) Overlapping Total an entity can belong to more than one lower-level entity type Example (STAR: SINGER and ACTOR) an entity must belong to one of the lower-level entity types Partial an entity need not belong to one of the lower-level entity types 2-47 Aggregation Consider the ternary relationship works-on Suppose we want to record managers for tasks performed by an employee at a branch 2-48
25 Aggregation (Conti.) Relationship sets works-on and manages represent overlapping information Every manages relationship corresponds to a works-on relationship However, some works-on relationships may not correspond to any manages relationships So we can t discard the works-on relationship Eliminate this redundancy via aggregation Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity 2-49 ER Diagram with Aggregation Without introducing redundancy, the following diagram represents: An employee works on a particular job at a particular branch An employee, branch, job combination may have an associated manager 2-50
26 Today s Agenda Introduction Entity Relationship odel Enhanced Entity Relationship odel Unified odel Language 2-51 UL UL: Unified odeling Language UL has many components to graphically model different aspects of an entire software system Use Case Activity Statechart Class UL Class Diagrams correspond to E-R Diagram, but there are several differences. 2-52
27 UL Class Diagram Notation 2-53 UL Class Diagrams Entity sets are shown as boxes, and attributes are shown within the box, rather than as separate ellipses in E-R diagrams. Binary relationship sets are represented in UL by just drawing a line connecting the entity sets. The relationship set name is written adjacent to the line. The role played by an entity set in a relationship set may also be specified by writing the role name on the line, adjacent to the entity set. The relationship set name may alternatively be written in a box, along with attributes of the relationship set, and the box is connected, using a dotted line, to the line depicting the relationship set. Non-binary relationships drawn using diamonds, just as in ER diagrams 2-54
28 UL Class Diagrams (Cont d) Cardinality constraints are specified in the form l..h, where l denotes the minimum and h the maximum number of relationships an entity can participate in. CHANGED: the positioning of the constraints is the same as the positioning of constraints on E-R diagrams in crow s foot notation. The constraint 0..* on the E2 side and 0..1 on the E1 side means that each E2 entity can participate in at most one relationship, whereas each E1 entity can participate in many relationships; in other words, the relationship is many to one from E2 to E1. Single values, such as 1 or * may be written on edges; The single value 1 on an edge is treated as equivalent to 1..1, while * is equivalent to 0..* UL Class Diagrams (Cont d) 0:1 0:N overlapping disjoint 2-56
29 Wrap-Up Conceptual odeling ERD Entity EERD Attribute Single/ultivalued attributes Identifier Relationship Degree Cardinality & Ordinality Generalization/Specialization (Inheritance) Aggregation UL Class Diagram 2-57 Next Week Introduction to Relational Data odel Foundations DDL part of SQL Logical Database Design Transforming E-R Diagrams into Relations Relational Algebra A formal query language for the relational data model Textbook Chapters 3 and Chapter 4 (without section 4.3) 2-58
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