|Freshers Resources||Data WareHousing||Computer Science||Analysis Design||Operating Systems|
- Data is an Important organizational asset
- Organizations require tools for effective management of their large and complex datasets
- Badly managed data is a liability - the cost of acquiring & managing it far exceeding the value derived from it
What is DBMS?
- A very large, integrated collection of data.
- Models real-world enterprise.
- Entities (e.g., students, courses)
- Relationships (e.g., Madonna is taking CS564)
- A Database Management System (DBMS) is a software package designed to assist in maintaining & utilizing large collections of data.
File System Vs DBMS
- Application using files need to swap large datasets between main memory and secondary storage
- Special programs would be needed to identify all data items
- Special code for different queries
- Multiple concurrent users could result in inconsistency
- Crash recovery
- Security and access control
Advantages of DBMS
- Data Independence
- Efficient data access
- Data integrity and security
- Data administration
- Concurrent access and Crash recovery
- Reduced application development time
Describing & storing data in DBMS
- A DBMS allows a user to define the data to be stored in terms of a data model
- A data model is a collection of high-level data description constructs that hide several lowlevel storage details
- The relational model of data is the most widely used model today.
–Main concept: relation, basically a table with rows and columns.
- A description and storage of data in terms of the data model is called a schema
- In the relational model, the schema for a relation specifies its name, name & type of each field
–Students(sid: string, name: string, login: string,age: integer, gpa:real)
·Application programs are insulated from changes in the way the data is structured and stored
·Data independence is achieved through use of three levels of data abstraction
Levels of abstraction
The data in a DBMS is described at three levels of abstraction
Levels of abstraction
- Views describe how users see the data.
- Conceptual(logical) schema defines logical structure
- Physical schema describes the files and indexes used.
Ø*Schemas are defined using DDL
Ø*Data is modified/queried using DML.
·In relational DBMS, the conceptual schema describes all relations that are stored in the database
ØStudents(sid: string, name: string, login: string, age: integer, gpa:real)
ØCourses(cid: string, cname:string, credits:integer)
ØEnrolled(sid:string, cid:string, grade:string)
·These relations contain information about entities, such as Students & courses and about relationships, such as Enrollment
·The physical schema summarizes how relations described in the conceptual schema are actually stored on secondary storage devices such as disks and tapes
·This schema decides :
ØThe file organizations to be used to store the relations
ØIndexes needed to speed up data retrieval operations
·External schema allows data access to be customized at the level of individual users or group of users
·Any given database has exactly one conceptual & physical schemas but it may have a number of external schemas
·Each external schema consists of a collection of one or more Views and relations from the conceptual schema
- Changes to Conceptual schema need not affect the users view of data - the definition
- of the view relation can be modified so that users get the same view as before
- Users can be shielded from changes in the logical structure of the data
- Logical data independence: Protection from changes in logical structure of data.
- The conceptual schema hides details such as how the data is actually laid out on the disk, the file structure, and the choice of indexes
- Physical data independence: Protection from changes in physical structure of data.
Architecture of DBMS
People who work with databases
- Database Implementors
- End users
- Database application developers
- Database administrator(DBA)
- DBA is responsible for:
ØDesign of the conceptual &physical schemas
ØSecurity & authorization
ØData availability & recovery from failures