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SQL Set Operators

SQL Set Operators   Traditional Set Operators: Union Compatibility:     Requirement for the traditional set operators.    Strong requirement.                         - Same number of columns.                         - Each corresponding column is compatible.                         - Positional correspondence.     Apply to similar tables by removing columns first. SQL UNION Example:           Example 1: Retrieve basic data about all university people.                     SELECT FacNo AS PerNo, FacFirstName                                      ...
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Database Three Level Architecture

 

Database Three Level Architecture

Objectives of Three-Level Architecture:

  • All users  should be able to access same data but have a different customized view. 
  • A user’s view is immune to changes made in other views Users should not need to know physical database storage details. 
  • DBA should be able to change database storage structures without affecting the user’s view. 
  • Internal structure of database should be un affected by changes to physical aspects of storage.
  • DBA should be able to change conceptual structure of database without affecting all users

ANSI-SPARC Three-Level Architecture:



External Level:

  • Users’ view of the database. 
  • Describes that part of database that is relevant to a particular user.
  • Different views may have different representation of same data (e.g. different date formats, age derived from DOB etc.)

Conceptual Level:

  • Community view of the database.
  • Describes what data is stored in the database and the relationships among the data. 
  • Along with any constraints on data.
  • Independent of any storage considerations.


Internal Level:

  • The physical representation of the database on the computer.
  • describes how the data is stored in the database.
  • physical implementation of the database to achieve optimal runtime performance and storage space utilization.
  • Data structures and file organizations used to store data on storage devices. 
  • Interfaces with the operating system access methods (file management techniques for storing and retrieving data records) to place the data on the storage devices, build the indexes, retrieve the data, and so on.


Differences between Three-Levels:







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SQL Set Operators

SQL Set Operators   Traditional Set Operators: Union Compatibility:     Requirement for the traditional set operators.    Strong requirement.                         - Same number of columns.                         - Each corresponding column is compatible.                         - Positional correspondence.     Apply to similar tables by removing columns first. SQL UNION Example:           Example 1: Retrieve basic data about all university people.                     SELECT FacNo AS PerNo, FacFirstName                                      ...
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