Lecture 1: Introduction to Database - Nubacad.com

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Levels of Abstraction (cont…) 1. Physical or Internal level The lowest level of abstraction describes how the data are actually stored. Complex low-level data structures and access methods are described in detail. 2. Logical or Conceptual level The middle level describes what data are stored in the database, and what relationships exist among those data. The entire database is described in terms of a small number of relatively simple structures. 3. View or External or User level The highest level of abstraction describes only part of the entire database of concern to a user or application program. application programs hide details of data types. Views can also hide information (e.g., salary) for security purposes. The system may provide many views for the same database. 21 • Conceptual schema: Example: University Database • Students(sid: string, name: string, login: string, age: integer, gpa:real) • Courses(cid: string, cname:string, credits:integer) • Enrolled(sid:string, cid:string, grade:string) • External Schema (View): • Course_info(cid:string,enrollment:integer) • Physical schema: • Relations stored as unordered files. • Index on first column of Students. View 1 View 2 View 3 Conceptual Schema Physical Schema DB 22 • Instance Instances and Schemas Database change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of the database. [ • Schema The overall design of the database is called the database schema. Database systems have several schemas, partitioned according to the levels of abstraction. At the lowest level is the physical schema; at the intermediate level is the logical schema; at the highest level is the subschema. 23 Data Models A data model is a collection of concepts for describing data and data relationships. •Entity-Relationship model •[ •Relational model •Other models: object-oriented model semi-structured data models (XML) Older models: network model and hierarchical model 24 6
Entity Relationship Model Models an enterprise as a collection of entities and relationships. Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects. Described by a set of attributes. Relationship: an association among several entities. Relational Model The relational model uses a collection of tables to represent both data and the relationships among those data. Each table has multiple columns, and each column has a unique name. teacherID name courseNo title teacher conduct course 25 26 Object oriented Data Model • Current generation systems have a need to handle complex data for complex applications such as - • computer aided design • computer aided software engineering • geographic information systems • interactive web sites • Object oriented databases are well suited to handling graphics based or multimedia applications. Can Stores objects (documents, multimedia, Image, Video). Semi-structured data models - Extensible Markup Language (XML) • Defined by the WWW Consortium (W3C) • Originally intended as a document markup language not a database language. • The ability to specify new tags, and to create nested tag structures made XML a great way to exchange data, not just documents. • XML has become the basis for all new generation data interchange formats. • A wide variety of tools is available for parsing, browsing and querying XML documents/data. 27 28 7
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Lecture 1: Introduction to Database - Nubacad.com

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