extended modelling in dbms using different.pptx
- 1. Organization of this Chapter: • Introduction • Superclasses and Subclasses • Specialization and Generalization • Constraints on specialization/generalization • Aggregation and Composition • Sample example February 21, 2025 1
- 2. Enhanced ER Model • The Enhanced ER model, EER model, includes all the modeling concepts of ER model along with some additional concepts like super class and sub class, specialization, generalization, aggregation, and composition. • Super class and Sub class: – Super class is an entity type that has a relationship with one or more subtypes. On the other hand, Sub class is a group of entities with unique attributes. – Sub class inherits properties and attributes from its super class. Sub class and Super class relationship leads the concept of Inheritance. – The relationship between sub class and super class is denoted with d symbol, where d means disjointness. February 21, 2025 2
- 4. Specialization and Generalization • Specialization – The process of designating sub groupings within an entity set is called Specialization. – An entity set may be specialized by more than one distinguishing features. – ER-design, specialization is depicted by an Inverted Triangle component labeled “IS A” (is a). – Specialization can be repeatedly used to refine a ER design. February 21, 2025 4
- 6. Generalization • Generalization is the process of combining similar entities into a more generalized entity. • It allows for the abstraction of common properties and relationships shared by multiple entities into a single higher-level entity. This process moves from specific to general. • Purpose: Simplifies the data model by grouping common features into a generalized (parent) entity, avoiding redundancy. • Hierarchy: Generalization forms a bottom-up hierarchy, where several lower-level entities are abstracted into a higher-level generalized entity. February 21, 2025 6
- 7. EER Model (contd..) • Generalization: – Generalization is a simple inversion of specialization. It is the process of extracting common properties from a set of entity types and creating a generalized entity type from it. – It is a bottom-up approach in which two or more entitiy types can be generalized to a higher level entity type, if they have some attributes in common. – Specialization adopts top-down approach, while Generalization adopts bottom-up approach. – A crucial property of the higher-level and lower-level entities created by specialization and generalization is attribute inheritance. – A lower-level entity set (or subclass) also inherits participation in the relationship sets in which its higher- level entity (or superclass) participates February 21, 2025 7
- 9. • Construct an ER diagram for the following: • “A university maintains record of students and the programmes in which they have enrolled. It stores the student name, id , phone no, address of student and programme code ,programme name and duration. A student is either full time or part time student(only one of the types). A student can register for multiple prgrammmes and vice versa.” February 21, 2025 9
- 10. Steps to draw ER diagram • An Entity-Relationship (ER) diagram is used to visually represent the structure of a database. It shows entities, relationships, and the attributes that describe them. Here’s a step-by-step guide to draw an ER diagram: • Identify Entities • Entities are objects or concepts about which data is stored. These could be things like "Customer," "Order," or "Product." • Entities are typically represented by rectangles. February 21, 2025 10
- 11. • Identify Relationships • A relationship is a connection between two or more entities. For example, a "Customer" can "Place" an "Order." • Relationships are shown using diamonds or lines connecting entities. • Example: • "Customer" places "Order" (relationship: places) February 21, 2025 11
- 12. • Determine the Cardinality of Relationships • Cardinality defines how entities relate to each other (e.g., one-to-one, one-to-many, many-to-many). • Use notations like "1" or "M" to specify cardinality. For example, one customer can place many orders (1 to M relationship). • Example: • 1 Customer places M Orders February 21, 2025 12
- 13. • Add Attributes • Attributes provide more details about each entity (e.g., "Customer Name," "Order Date"). • Attributes are represented by ovals connected to their respective entity rectangles. • Example: • Attributes of "Customer": CustomerID, CustomerName, Email • Attributes of "Order": OrderID, OrderDate, TotalAmount February 21, 2025 13
- 14. • Identify Primary Keys • Each entity should have a primary key, a unique identifier for each record in that entity • Define Foreign Keys (if any) • Foreign keys establish links between related entities. For example, "Order" might have a foreign key "CustomerID" that links it back to the "Customer" entity. • Example: • The "Order" entity could have a "CustomerID" attribute, which is a foreign key from the "Customer" entity. February 21, 2025 14
- 15. • Normalize the Diagram (Optional) • Review your diagram to ensure that there is no redundancy or unnecessary relationships. • Normalize the design to avoid data duplication. February 21, 2025 15
- 16. Constraints in Generalization: • Loss of Specificity: By generalizing, you may lose some specific characteristics of individual entities in the higher-level abstraction • Design Complexity: While generalization reduces redundancy, it may increase the complexity of relationships and hierarchy in the database schema. • . February 21, 2025 16
- 17. Constraints on Generalization/Specialization • Membership Constraint – Condition defined/Attribute defined • All the lower level entity is identified on the basis of same type of attribute this type of generalization is said to be attribute defined. – Here the lower level entity is generalized on the basis of accnt type. February 21, 2025 17 Accnt no Accnt name Branch Accnt type 005 a1 Ambala Saving a 008 a2 chandigarh Current a 009 a3 mohali Saving a
- 18. – User-defined • User-defined lower-level entity sets are not constrained by a membership condition; rather, the database user assigns entities to a given entity set. • EX: A celebrity can be personalised into actor and politician. • Disjoint constraints – Disjoint: • The disjoint constraint only applies when a superclass has more than one subclass. If the subclasses are disjoint, then an entity occurrence can be a member of only one of the subclasses .e.g. postgrads or undergrads; one cannot be both. • To represent a disjoint superclass/subclass relationship, Or is used. February 21, 2025 18
- 19. contd.. • Overlapping: This applies when an entity occurrence may be a member of more than one subclass, e.g. student and staff; some people are both. And is used to represent the overlapping specialization/generalization relationship in the ER diagram. February 21, 2025 19
- 20. contd.. • Completeness constraints – Total: Each superclass (higher-level entity) must belong to subclasses (lower-level entity sets), e.g. a student must be postgrad or undergrad. To represent completeness in the specialization/generalization relationship, the keyword Mandatory is used. February 21, 2025 20
- 21. contd.. • Partial: Some superclasses may not belong to subclasses (lower-level entity sets), e.g. some people at UCT are neither student nor staff. The keyword Optional is used to represent a partial specialization/generalization relationship. February 21, 2025 21
- 22. Aggregation and Composition • Aggregation: – Aggregation represents a has-a relationship between entity types, where one represents the whole and the other the part. – An example of aggregation is the Car and Engine entities. A car is made up of an engine. The car is the whole and the engine is the part. – Aggregation does not represent strong ownership. This means, a part can exist on its own without the whole. There is no stronger ownership between a car and the engine. An engine of a car can be moved to another car. – A line with a diamond at the end is used to represent aggregation.The whole must be put at the end of the diamond. February 21, 2025 22
- 23. contd.. • Composition – Composition is a form of aggregation that represents an association between entities, where there is a strong ownership between the whole and the part. – For example,: a tree and a branch have a composition relationship. A branch is 'part' of a 'whole' tree - we cannot cut the branch and add it to another tree. – A line with a filled diamond at the end is used to represent composition where the diamond side is towards the part side. February 21, 2025 23
- 24. An Example February 21, 2025 24