1. Views-basics 1

2. 2 Introduction a view is a perspective of the database different users may need to see the database differently; this is achieved through the view mechanism a view is part of the external level remember the three level architecture: internal, conceptual and external

3. Views-basics 3 Introduction view = a named relational expression suppose Employee ( Emp_Id, Name, Address, Salary, Dept_id ) Department (Dept_id, Manager, Budget, Office ) then views can be defined as SELECT Name, Salary, Dept_id FROM Employee WHERE Salary > 35 SELECT Name, Employee.Dept_id FROM Employee, Department WHERE Budget > 1500 AND Employee.Dept_id = Department.Dept_id

4. Views-basics 4 View - a window to the database the database - a set of base tables

5. Views-basics 5 View named expression of relational algebra (calculus) relational closure virtual relation substitution process

6. Views-basics 6 Data definition for views in SQL CREATE VIEW [ ] AS [WITH CHECK OPTION] ; DROP VIEW ; ::= RESTRICT | CASCADE

7. Views-basics 7 SQL views - vertical and horizontal --vertical view CREATE VIEW Emp AS SELECT Emp_Id, Name, Address, Dept_id FROM Employee ; --supposing that the salary is confidential --horizontal view CREATE VIEW GoodEmp AS SELECT Emp_id, Name, Address, Salary, Dept_id FROM Employee WHERE Salary > 45 ;

8. Views-basics 8 SQL views - join --an employee is safe if s/he works for a rich department CREATE VIEW SafeEmployees AS SELECTName, Employee.Dept_id FROM Employee, Department WHERE Budget > 1500 AND Employee.Dept_id = Department.Dept_id

9. Views-basics 9 SQL views - aggregate functions CREATE VIEW TotSalPerDept AS SELECTDept_id, SUM(Salary) AS TotSal FROM Employee GROUP BY Dept_id

10. Views-basics 10 Using views views are used as if they were base relations from the point of view of the user, a view is the same as a base relation however, certain restrictions exist e.g. see next slide

11. Views-basics 11 SQL restrictions on views in a view, a column that is based on an aggregate function cannot be subject to an aggregate function or to a WHERE clause (e.g. TotSal before) a grouped view may never be joined with a base table or another view

12. Views-basics 12 WITH CHECK OPTION only for updateable views migrating rows a row of a view, after being updated, may not satisfy the condition of the view anymore, therefore it will migrate out of the view WITH CHECK OPTION avoids such situations; the update is not permitted if the row will no longer satisfy the condition of the defining query

13. Views-basics 13 View resolution view = virtual relation view = expression that is evaluated every time it is used evaluating an expression on a view = view resolution view resolution substitution

14. Views-basics 14 Activity Consider a view definition and a query on this view. Explain how (do you think) the expression is evaluated.

15. Views-basics 15 Advantages logical data independence users and user programs are immune to changes in the logical structure of the DB to what extent logical data independence can be guaranteed? restructuring tables : add/delete columns, change key definition, change integrity constraints, rename columns/tables, split tables,...

16. Views-basics 16 Advantages automatic/improved security reduced complexity through macro facility customisation the same data can be seen differently by users through macro facility data integrity –WITH CHECK OPTION

17. Views-basics 17 Disadvantages update restriction will see in more detail next lecture structure restriction performance

18. Views-basics 18 Activity Find out, through experiments, to what extent logical data independence is provided in Postgres some changes on base relations or on the views used in other views definitions can be made without requiring the redefinition of the views some other changes require view redefinition, but the structure (and name) of the view can stay the same