What do you mean by logical data independence?The ability to change the logical schema without changing theexternal schema or application programs is called as LogicalData Independence. ORThe ability to change the logical schema without having tochange the external schema.
ExamplesThe addition or removal of newentities, attributes, or relationships to theconceptual schema should be possible withouthaving to change existing external schemas orhaving to rewrite existing application programs.
What do you mean by Physical Data Independence The ability to change the physical schema without changing the logical schema is called as Physical Data Independence. Changes in the physical schema may include. Using new storage devices. Using different data structures. Switching from one access method to another. Using different file organizations or storage structures. Modifying indexes.
ExamplesA change to the internal schema, such as usingdifferent file organization or storagestructures, storage devices, or indexingstrategy, should be possible without having to changethe conceptual or external schemas.
Elements of E-R Model In the E/R model, the structure of data is represented graphically, as an “entity-relationship diagram,” using three Principal element types: Entity Sets. An entity is an abstract object of some sort, and a collection of similar entities forms an entity set. Attributes The properties of the entities in the set. Relationship The connections among two or more entity Sets
Example of Elements of E-R Model Entity Sets Departments Professors Students Administrators Attributes Name of Departments, Phone No., Address... Name, SSN, Address of Professors... Relationship Students and Professors are under a certain department Admin manage the campus/ departments
Attribute An attribute is an item of data held about an entity In computer systems an attribute is a field of information Example : Sales System Entity : Customer Attributes : Customer Name, Customer Address, Customer Phone Number
A key attribute uniquely identifies a specific occurrence of an entity This may also be referred to as a primary key Examples of primary keys include: ISBN Student Number Staff IDFor Example In a Sales System, customer name and address are attributes of the entity customer. The primary key is likely to be Customer Code or Customer ID as the unique identifier for each customer in the system.
Example: A library database contains a listing of authorsthat have written books on various subjects (one author perbook). It also contains information about libraries that carrybooks on various subjects. Entity sets: authors, subjects, books, libraries Relationship sets: wrote, carry, indexed Subject title isbn matter SS# authors wrote books index subject name quantity carry address libraries