Published on March 15, 2014
Physical and Logical data Independence
Logical Data Independence
What do you mean by logical data independence? The ability to change the logical schema without changing the external schema or application programs is called as Logical Data Independence. OR The ability to change the logical schema without having to change the external schema.
Examples The addition or removal of new entities, attributes, or relationships to the conceptual schema should be possible without having to change existing external schemas or having to rewrite existing application programs.
Physical Data Independence
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.
Examples A change to the internal schema, such as using different file organization or storage structures, storage devices, or indexing strategy, should be possible without having to change the 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 ID 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. For Example
Example: A library database contains a listing of authors that have written books on various subjects (one author per book). It also contains information about libraries that carry books on various subjects. Entity sets: authors, subjects, books, libraries Relationship sets: wrote, carry, indexed carry books indexwrote subjectauthors SS# name title libraries quantity address isbn Subject matter
Entity-Relationship (E-R) Model COURSE# CTITILE PROF SCHED ROOM COURSE ENROLL STUDENT STUID SNAME MAJOR CREDITS GRADE
Example of the 3 elements in Entity Relationship Diagram Entity B Relationship Attribute Entity C Entity A Attribute Attribute
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Logical Data Independence : The ability to change the logical schema without changing the external schema or application programs is called as.
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The separation of data from the programs that use the data. Nearly all modern applications are based on the principle of data independence.