The University of Toronto has several departments Each department is managed by a chair and at least one professor Professors must be assigned to one but possibly more departments At least one professor teaches each course but a professor may be on sabbatical and not teach any course Each course may be taught more than once by different professors We know of the department name the professor name the professor employee id the course names the course schedule the term year that the course is taught the departments the professor is assigned to the department that offers the course Practice Example Tutorial Week 7 Class and Entity Relationship Diagrams 1 Class Diagrams and Entity Relationship Diagrams ERD Class diagrams and ERDs both model the structure of a system Class diagrams represent the dynamic aspects of a system both the structural and behavioural features ERDs depicting only structural features provide a static view of the system 2 Class Diagrams 2 1 Elements of a class diagram 2 1 1 class A class is a general concept represented as a square box A class defines the structural attributes and behavioural characteristics of that concept Shown as a rectangle labeled with the class name Class Name 2 1 2 association Class 1 Association Class 2 A semantic relationship between classes A line that joins two classes Food Simple association between two classes A solid triangle with the association name indicates the direction in which the association is meant to be read Class 2 n ary Association expresses an association between multiple classes Member has a relationship 2 1 2 1 binary Person Eats 2 1 2 2 n ary Class 1 n ary Class 3 2 1 2 3 Aggregation Team page 1 of 14 Tutorial Week 7 Class and Entity Relationship Diagrams 2 1 2 4 Composition Car is composed of relationship Engine 2 1 2 5 Generalization Car is a kind of relationship Volvo 2 1 2 6 Dependency The source class depends on uses the target class not used for requirements analysis Project Project Manager Team 2 1 2 7 Realization Class supports all operations of target class but not all attributes or associations not used for requirements analysis datatype Human Resources datatype Project Manager Worker datatype System Administrator page 2 of 14 Tutorial Week 7 Class and Entity Relationship Diagrams 2 1 2 8 Association Classes Can be applied to binary or n ary associations Used to capture the attributes of an association Association Class class 2 class 1 2 1 3 attribute What is known about each object of this class type Typical characteristics class 1 attribute 1 attribute 2 attribute 3 2 1 4 operation What objects of this class type do Typical actions that are relevant to the current analysis class 1 attribute 1 attribute 2 attribute 3 operation 1 operation 2 operation 3 3 Creating a Class Diagram Here are some steps to help create a class diagram 3 1 Identify Classes These are the abstract or physical things in our system which we wish to describe Find all the nouns and noun phrases in the domain descriptions you have obtained through your analysis Consider these class candidates 3 2 Find Associations Now find the verbs that join the nouns e g The professor noun teaches verb students noun The verb in this case defines an association between the two nouns Identify the type of association Use a matrix to define the associations between classes 3 3 Draw Rough Class Diagram Put classes in rectangles and draw the associations connecting the classes 3 4 Fill in Multiplicity Determine the number of occurrences of one class for a single occurrence of the associated class page 3 of 14 Tutorial Week 7 Class and Entity Relationship Diagrams 3 5 Identify Attributes Name the information details fields which are relevant to the application domain for each class 3 6 Identify Behaviours Specify the operations that are required for each class assume getter and setter methods for each attribute 3 7 Review your diagram and fine tune it Look for inconsistencies and errors Fix them Make sure you have captured everything required from the domain you are studying that your diagram is complete 4 Entity Relationship Diagrams 4 1 Elements of Entity Relationship Diagrams 4 1 1 Entity A data entity is anything about which we want to store data 4 1 2 Relationship A relationship is a semantic association between one or more entities 4 1 3 Attribute Key Attribute A data attribute is a property common to an entity Attribute 1 Entity 1 Attribute 2 Attribute 3 4 2 Creating an ERD Here are the steps you may follow to create an entity relationship diagram page 4 of 14 Tutorial Week 7 Class and Entity Relationship Diagrams 4 2 1 Identify Entities Identify the entities These are typically the nouns and noun phrases in the descriptive data produced in your analysis Do not include entities that are irrelevant to your domain 4 2 2 Find Relationships Discover the semantic relationships between entities These are usually the verbs that connect the nouns Not all relationships are this blatant you may have to discover some on your own The easiest way to see all possible relationships is to build a table with the entities across the columns and down the rows and fill in those cells where a relationship exists between entities 4 2 3 Draw Rough ERD Draw the entities and relationships that you have discovered 4 2 4 Fill in Cardinality Determine the cardinality of the relationships You may want to decide on cardinality when you are creating a relationship table 4 2 5 Define Primary Keys Identify attribute s that uniquely identify each occurrence of that entity 4 2 6 Draw Key Based ERD Now add them the primary key attributes to your ERD Revise your diagram to eliminate many to many relationships and tag all foreign keys 4 2 7 Identify Attributes Identify all entity characteristics relevant to the domain being analyzed 4 2 8 Map Attributes Determine which to entity each characteristic belongs Do not duplicate attributes across entities If necessary contain them in a new related entity 4 2 9 Draw fully attributed ERD Now add these attributes The diagram may need to be modified to accommodate necessary new entities 4 2 10 Check Results Is the diagram a consistent and complete representation of the domain page 5 of 14 Tutorial Week 7 Class and Entity Relationship Diagrams 5 Practice Example The University of Toronto has several departments Each department is managed by a chair and at least one professor Professors must be assigned to one but possibly more departments At
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