DBMS DATA VIEW | Readstall


13-12-18 Gudditi Naganjaneyulu 0 comment

DBMS  data :

Data abstraction  :

The data in a DBMS is described at three levels of abstraction . the database that consists of a schema at each of these 3 levels of abstraction

Conceptual levels

Physical levels

External levels

A data definition language (DDL)isused to defines the external and conceptual schemas

All DBMS suport SQLcommands that aspects the physical schema, but thses command are not includedin sql standard System catalaguestores all the infomation about conceptual external and physical schemas

levels of data abstraction 

Conceptual shema :

The conceptual schema /logical schema describes about data that has been stores in dbms in terms of data models in DBMS the conceptual schema describes all the relations of the data that has been stored in database

Eg : conceptual schema :-

 Student(sid : integer ,name: string ,login : string , age : integer , gpa: real );

Physical schema

 The physical schema that describes how the relations in conceptual schema are actually stored on secondary storage devices ,such as disks and tapes

  The physical schema specifies additional additional storage details and specifies and index are bases

Fg : a relations stored as unordered files

   index on first column of students

External schema  :

   Each external schema consists of a collections of one or more views and relations from the conceptual schema . navies is conceptually a relation ,but the records in a view are not stored in the DBMS

External schema design is guided by end user requirement

Eg :

Course info (aid : string ,flame .string,enrollment ,integer )

Database instances and sch-mas

  Schema: schema is the selection structure of the database ot the overall design of the database i.e logical structure .a schema is collection of database objects of a user

  • A database schema is the selection structure that represents that represents the view of the entire database
  • It defines how the data is organised and how the relations among them are associated
  • It’s the database designers who design the schema to help programmers understand the database and make it useful

Eg student relation

The database consists of information about aset of customers and accounts and the relationship between them

Database schema

A database schema can be catageorised into two types

Physical  database schema

The physical database schema actually stores the data in the form of files , indicates  etc… It defines how the data will be stored in a secondary storage

Logical database chema

This schema definesall the logical constrants that need to be applied on the data stored .itdefines tables views and integrity conststains

Databse instancy 

 Database instances is the collections of information stores in the database at rhe a particular moment .databse schema is as selection that dfatabse designers have imposed .it consists of the following

Logical data is the data about databsase that ia stores information about how data is managed iside…

Eg : employee table (emp table )

Physical data independence :

 All the schemas are logiacal and the actual data stores in the form of bits in the disk .it changes physical data without impacting the schema or logical data

Eg : how emp table data stores in hard disk 

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