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Creating views is easy. You can create views using most SELECT statements, including those made with common table expressions. For example, you might want to create a view that lists all the customers in the Sales.Customer table from the AdventureWorks2008 database along with their names from the Person.Person table. You could use that view instead of the Sales.Customer table in other queries. Here is the syntax to create, alter, and drop views: CREATE VIEW <view name> AS SELECT <col1>, <col2> FROM <table> ALTER VIEW <view name> AS SELECT <col1>, <col2> FROM <table> DROP VIEW <view name> Type in and execute the code in Listing 8-7 to learn how to create and use views. Listing 8-7. Creating and Using a View USE AdventureWorks2008; GO --1 IF OBJECT_ID('dbo.vw_Customer') IS NOT NULL BEGIN DROP VIEW dbo.vw_Customer; END; GO
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CHAPTER 8 MOVING LOGIC TO THE DATABASE
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--2 CREATE VIEW dbo.vw_Customer AS SELECT c.CustomerID, c.AccountNumber, c.StoreID, c.TerritoryID, p.FirstName, p.MiddleName, p.LastName FROM Sales.Customer AS c INNER JOIN Person.Person AS p ON c.PersonID = p.BusinessEntityID GO --3 SELECT CustomerID,AccountNumber,FirstName, MiddleName, LastName FROM dbo.vw_Customer; GO --4 ALTER VIEW dbo.vw_Customer AS SELECT c.CustomerID,c.AccountNumber,c.StoreID, c.TerritoryID, p.FirstName,p.MiddleName, p.LastName, p.Title FROM Sales.Customer AS c INNER JOIN Person.Person AS p ON c.PersonID = p.BusinessEntityID GO --5 SELECT CustomerID,AccountNumber,FirstName, MiddleName, LastName, Title FROM dbo.vw_Customer ORDER BY CustomerID; Figure 8-10 shows the results. Code section 1 drops the view if it already exists. Code section 2 creates the view. Notice that the GO statements surround the CREATE VIEW code so that it has its own batch. Any time you create or alter a view, the code must be contained within a batch that has no other code except for comments. Notice that the view begins with the characters vw_ designating that it is a view, not a table. Often companies will have naming conventions such as this; be sure to find out whether your shop requires special naming for views. Statement 3 selects several of the columns from the view. At this point, you could include a WHERE clause, include an ORDER BY clause, or involve the view in an aggregate query if you wanted. Basically, you can treat the view like a table in a SELECT statement. Statement 4 alters the view, adding a column. Statement 5 is another SELECT statement, which includes the new column and an ORDER BY clause.
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Figure 8-10. The results of creating and using a view To see the view in SQL Server Management Studio, navigate to the Views section of the AdventureWorks2008 database. You will see the newly created view along with several views that ship with the database. Notice that each of the preexisting views belongs to one of the schemas in the database. Just like tables, you can script out the definition or bring up a graphical designer. Figure 8-11 shows the graphical designer for the view.
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Figure 8-11. The graphical designer for views
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Avoiding Common Problems with Views
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You may decide that you should set up views joining all the tables in the database and just write queries against those views, but there are often problems associated with using views as the main strategy for database development. One problem is the confusion that can result from views created on top of other views. Tracking down logic errors becomes difficult when they are buried in layers of views. If the underlying table structure changes, the view may produce strange results, as shown in Listing 8-8. Make sure that you do not use views in a way that will negatively impact performance. For example, suppose you created a view containing the customers, customer addresses, sales, and sales details tables. If you just wanted a list of customers, you could query the view but would be also accessing tables you did not need to view at that time. Developers often want to add an ORDER BY clause to a view definition. This is actually disallowed except under specific conditions and does not make sense because you can always add the ORDER BY clause to the outer query. (To add an ORDER BY to a view definition, you must add the TOP specifier to the view. You will learn about TOP in 10.) In fact, the database engine does not guarantee that the data will be returned in the order specified in the view definition according to the CREATE VIEW topic in SQL Server Books Online. Type in and execute the code in Listing 8-8 to learn more about two common problems with views. Listing 8-8. Common Problems Using Views USE AdventureWorks2008; GO --1 IF OBJECT_ID('vw_Dept') IS NOT NULL BEGIN DROP VIEW dbo.vw_Dept; END; IF OBJECT_ID('demoDept') IS NOT NULL BEGIN DROP TABLE dbo.demoDept; END; --2 SELECT DepartmentID,Name,GroupName,ModifiedDate INTO dbo.demoDept FROM HumanResources.Department; GO --3 CREATE VIEW dbo.vw_Dept AS SELECT * FROM dbo.demoDept; GO
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