COMP 244. ER-Diagram Notations. Entity-Relationship Diagrams DATABASE CONCEPTS & APPLICATIONS. Database Concepts & Applications 1.

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1 COMP 244 DATABASE CONCEPTS & APPLICATIONS ER-Diagram Notations Attribute Key Attribute Multi-valued attributes Entity-Relationship Diagrams Derived Attribute Weak Entity Identifying Relationship 1 2 Database Concepts & Applications 1

2 Entities and Attributes Entity sets are shown by rectangles. The entity name is shown inside the rectangle Entities and Attributes Attributes can be attached to entities in two ways: Number Name Location Num All entities must have attributes. contains Number Name contains Location Num 3 4 Database Concepts & Applications 2

3 Relationships Entities can have relationships. Relationships are shown by diamonds. There are two ways to represent relationship names: Relationship Attributes Relationships can have descriptive attributes. Descriptive attributes record information about relationships Occupies id Name Rent Number Size Occupies Occupies 5 6 Database Concepts & Applications 3

4 Relationship Cardinalities The maximum cardinality represents the maximum number of instances of each entity that participate in the relationship N Occupies N:1 Occupies 1 Relationship Cardinalities The minimum cardinality indicates whether entity instances are required to exist or not. Example: A student must have a room. A room can be occupied by students (optional). Sometimes, rooms can be empty. A student must take courses. A course can be taken by students (i.e., we can have courses that are not yet offered to students). 7 8 Database Concepts & Applications 4

5 Relationship Cardinalities The minimum cardinality is represented by a hash mark on the line that connects the entity that must exist and an oval on the line that connects the entity that may or may not exist. Recursive Relationships A unary relationship is called a recursive relationship. A recursive relationship is represented by a curve that connects an entity class with itself. N Occupies 1 1:N mate 9 10 Database Concepts & Applications 5

6 Weak Entities A weak entity is an entity that cannot exist in the database by itself (i.e., without having a relationship with another entity (its owner)). A strong entity can exist independently. Example: In a university, a Course is a strong entity and a Course_Section can be modeled as a weak entity. Employees can purchase insurance policies for their dependents. So, Employee is a strong entity and Dependent is a week entity. Weak Entities The strong entity and weak entity must participate in a one-to-many relationship (i.e., one owner, many weak entities). We depict a weak entity set by double rectangles. Course 1 has N Course_Section Database Concepts & Applications 6

7 Weak Entities A weak entity cannot be identified uniquely only by considering its own attributes For a given owner, a weak entity will have a discriminator (or partial key) that uniquely identifies all the entity instances that depend on that owner. For a given Course we can identify all Course_Section by their semester (including year) and section_number (if there is more than one section). For a given Employee we can identify all Dependent instances by their name. 13 Example: Weak Entities For a given Employee we can identify all Dependent instances by their name Employee ID Ahmed Khalid Raya Dependents 14 Database Concepts & Applications 7

8 Weak Entities The primary key of a weak entity is formed by combining the primary key of the strong entity (owner) on which the weak entity s existence depends, plus the weak entity set s discriminator. The primary key is (id, name) id name phone cost name The partial key is identified by a dotted line age Weak Entities Example: For a given Loan we can have many Payment. The discriminator for Payment is payment_number. The primary key for Payment is (loan_number, payment number). loan_number Loan amount Loan_Payment payment_number Payment payment_date Employees Policy Dependents The thick line indicates total participation Database Concepts & Applications 8

9 ID-Dependent Entities An ID-dependent entity is the one in which its identifier includes the identifier of another entity. It is similar to relationships between weak and strong entities. Example: Building (BuildingName) Appartment (BuildingName, AppartmentNumber) Appartment is physically and logically dependent on Building ID-Dependent Entities Other Examples: Product and Version Product [ProductName] Version [ProductName, ReleaseNumber] Textbook and Edition Textbook [Title] Edition [Title, EditionNumber] Database Concepts & Applications 9

10 ID-Dependent Entities Other Examples: Advisor and Advisor [AdvisorCode] [ID, AdvisorCode] is logically and physically NOT dependent on Advisor CONCLUSIONS: is not a weak entity. All weak entities must logically depend on another entity. All ID-Dependent entities are weak entities. Subtype Entities Entity Sets can be classified into hierarchies. Assume we have the entity. A can be undergraduate or graduate. These are called subtypes Database Concepts & Applications 10

11 Subtype Entities We need to store additional information depending on the subtype: For undergraduate, we need to store HighschoolGrade and MonthlyAllowance. For graduate, we need to store Tuition and ResearchTopic. Subtype Entities We can record information about an entity and all its subtypes in two ways: We can have only the main entity and in it we include all attributes. contains How to represent these information?? 21 ID Name Major HighschoolGrade MonthlyAllowance Tuition ResearchTopic 22 Database Concepts & Applications 11

12 Method 2: In the main entity, we only record the attributes that are common to all subtypes. In each subtype, we record the attributes that are specific to that entity. Subtype Entities contains ID Name Major Undergraduate Undergraduate contains ε HighschoolGrade MonthlyAllowance This is called a Generalization- Specialization or IS-A relationship. ε Supertype Graduate Subtype Graduate contains Tuition ResearchTopic 23 IS-A Relationship The supertype and all subtype entities share a common identifier (for example,, Undergraduate, and Graduate entities have Number as an identifier). The supertype is the general and the subtype is the specialized. Generalization-Specialization hierarchies have a special characteristic called inheritance. 24 Database Concepts & Applications 12

13 Overlap Constraints determine whether two subclasses are allowed to contain the same entity. IS-A Relationship hourly_wages ssn hours_worked Hourly_Emp name Employee ISA lot contractid Example: An employee cannot be both a Contract Employee and an Hourly Employee Example: An employee can be both a Contract Employee and a Senior Employee Contract_Emp IS-A Relationship Covering Constraints determine whether the entities in the subclasses collectively include all entities in the superclass. Example: Not ALL employees need to be Contract Employee and Hourly Employee Example: ALL college students are either Undergraduate or Graduate Database Concepts & Applications 13

14 Why need to use ISA relationship? To add descriptive attributes that make sense only to specific entity instances For example: Thesis title is only relevant to graduate students To identify entities that can participate in specific relationships For example: We might want to select department managers only from senior employees Why need to use ISA relationship? To identify entities that can participate in specific relationships For example: We might want to select department managers only from senior employees Junior Employee Employee ISA Senior Employee Manages Department Database Concepts & Applications 14

15 Aggregation Aggregation is used to express relationships among relationships. Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships. Ternary relationships are not sufficient to replace aggregation because of the attributes that are normally associated with each relationship. Departments Aggregation Employees Monitors Sponsors Each sponsership is monitored by at most one Employee Projects Database Concepts & Applications 15