1. Implementing Views
Advanced Database
Dr. AlaaEddin Almabhouh

2. Topic & Structure of Lesson
What Is a View?
Types of Views
Advantages of Views
Syntax for Creating Views
Performance Considerations for Views
Slide 2 (of 33)

3. What Is a View?
Dynamic result of one or more tables operating on base relations to produce another relation.
Virtual relation that does not necessarily actually exist in the database but is produced upon request, at time of request.
The rows and columns of data come from tables referenced in the query defining the view and are produced dynamically when the view is referenced.
Common examples of views are:
A subset of rows or columns of a base table.
A union of two or more base tables.
A join of two or more base tables.
A statistical summary of a base table.
A subset of another view, or some combination of views and base table

4. What Is a View?
Contents of a view are defined as a query on one or more base relations.
With view resolution, any operations on view are automatically translated into operations on relations from which it is derived.
With view materialization, the view is stored as a temporary table, which is maintained as the underlying base tables are updated.
Slide 31 (of 82)

5. What Is a View? Employee (table) vEmployee (view) EmployeeID LastName
FirstName
Title …
287
Mensa-Annan
Tete
Mr.
288
Abbas
Syed
289
Valdez
Rachel
NULL
vEmployee (view)
LastName
FirstName
Mensa-Annan
Tete
Abbas
Syed
Valdez
Rachel

6. Types of Views Standard views Indexed views Partitioned views
Combine data from one or more base tables (or views) into a new virtual table
Indexed views
Materialize (persist) the view through the creation of a unique clustered index on the view
Partitioned views
Join horizontally partitioned data from one or more base tables across one or more servers

7. Syntax for Creating Views
Use CREATE VIEW statement:
Restrictions:
Cannot nest more than 32 levels deep
Cannot contain more than 1,024 columns
Cannot use COMPUTE, COMPUTE BY, or INTO
Cannot use ORDER BY without TOP
CREATE VIEW [ schema_name.] view_name [ (column [ ,...n ] ) ] [WITH [ENCRYPTION] [SCHEMABINDING] [VIEW_METADATA] ] AS select_statement [ ; ] [ WITH CHECK OPTION ]

8. Sample for Creating Views
CREATE VIEW Orders_View AS
SELECT OrderID, OrderDate, o.CustomerID, CompanyName,Address, Phone
FROM dbo.Orders o JOIN dbo.Customers c on o.CustomerID = c.CustomerID
WHERE Country = ‘USA’

9. Syntax for Altering and Dropping Views
Alter by using the ALTER VIEW Transact-SQL statement:
Drop by using the DROP VIEW Transact-SQL statement:
ALTER VIEW [ schema_name.]view_name [ (column [ ,...n ] ) ] [WITH [ENCRYPTION] [SCHEMABINDING] [VIEW_METADATA] ] AS select_statement [ ; ] [ WITH CHECK OPTION ]
DROP VIEW [ schema_name.]view_name [ ...,n ] [ ; ]

10. View Encryption
Use the WITH ENCRYPTION option on CREATE VIEW Transact-SQL statement
Encrypts view definition in sys.syscomments table
Protects view creation logic
CREATE VIEW [HumanResources].[vEmployee] WITH ENCRYPTION AS SELECT e.[EmployeeID],c.[Title],c.[FirstName],c.[MiddleName] ,c.[LastName],c.[Suffix],e.[Title] AS [JobTitle] ,c.[Phone],c.[ Address] FROM [HumanResources].[Employee] e INNER JOIN [Person].[Contact] c ON c.[ContactID] = e.[ContactID]
Use WITH ENCRYPTION on ALTER VIEW statements to retain encryption

11. Considerations for Modifying Data in a View
Views do not maintain a separate copy of data
Updates to views modify base tables
Restrictions:
Cannot affect more than one base table
Cannot modify columns derived from aggregate functions or calculations
Cannot modify columns affected by GROUP BY, HAVING, or DISTINCT clauses
Updates to views are restricted by using the WITH CHECK OPTION

12. Performance Considerations for Views
Views introduce performance overhead because views are resolved dynamically
Nested views introduce risk of performance problems
Review definition of unencrypted nested views
Use SQL Server Profiler to review performance
Indexed views and partitioned views can improve performance

13. What Is an Indexed View? A view with a unique clustered index
Materializes view, improving performance
Allows query optimizer to use view in query resolution
Use when:
Performance gains outweigh maintenance overhead
Underlying data is modified infrequently
Queries perform a significant number of joins and aggregations
CREATE UNIQUE CLUSTERED INDEX [IX_vStateProvinceCountryRegion] ON [Person].[vStateProvinceCountryRegion] ( [StateProvinceID] ASC, [CountryRegionCode] ASC)

14. What Is a Partitioned View?
Joins horizontally partitioned data from a set of tables across one or more servers
SQLServerNorth.Sales.Sale
SQLServerSouth.Sales.Sale
CREATE VIEW vSales AS SELECT * FROM SQLServerNorth.Sales.Sale
UNION ALL SELECT * FROM SQLServerSouth.Sales.Sale
vSales

15. Restrictions on Views
SQL imposes several restrictions on creation and use of views.
If column in view is based on an aggregate function:
Column may appear only in SELECT and ORDER BY clauses of queries that access view.
Column may not be used in WHERE nor be an argument to an aggregate function in any query based on view.
Slide 44 (of 82)

16. Restrictions on Views For example, following query would fail:
SELECT COUNT(cnt)
FROM StaffPropCnt;
Similarly, following query would also fail:
SELECT *
FROM StaffPropCnt
WHERE cnt > 2;
Slide 45 (of 82)

17. Restrictions on Views
Grouped view may never be joined with a base table or a view.
For example, StaffPropCnt view is a grouped view, so any attempt to join this view with another table or view fails.
Slide 46 (of 82)

18. View Updatability
All updates to base table reflected in all views that encompass base table.
Similarly, may expect that if view is updated then base table(s) will reflect change.
Slide 47 (of 82)

19. View Updatability
If change definition of view and replace count with actual property numbers:
CREATE VIEW StaffPropList (branchNo,
staffNo, propertyNo)
AS SELECT s.branchNo, p.propertyNo
FROM Staff s, PropertyForRent p
WHERE s.staffNo = p.staffNo;
Slide 49 (of 82)

20. View Updatability Now try to insert the record:
INSERT INTO StaffPropList
VALUES (‘B003’, ‘PG19’);
Still problem, because in PropertyForRent all columns except postcode/staffNo are not allowed nulls.
However, have no way of giving remaining non-null columns values.
Slide 50 (of 82)

21. Advantages of Views Focus the data for a user Mask database complexity
Simplify management of user permissions
Improve performance
Organize data for export to other applications

22. Disadvantages of Views
Update restriction
Structure restriction
Performance
Slide 60 (of 82)

23. Q & A
Slide 81 (of 82)