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Although you can define a view from more than one table, any data modifications exe cuted through the view are restricted to a single table. All changes have to directly ref erence columns, not derivations of a column. Columns cannot be derived through the following:
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Aggregate functions such as AVG, COUNT, SUM, MIN, and MAX. A computation that involves other columns or operations on a column such as SUBSTRING. Columns generated by using operators such as UNION, CROSSJOIN, and INTERSECT. The view definition cannot contain a GROUP BY, HAVING, or DISTINCT clause. TOP cannot be used when WITH CHECK OPTION is also specified.
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Although views can be used to insert, update, and delete data, views are almost never used for those purposes. Stored procedures are always a better option because changes can be more easily validated as well as being more flexible.
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Views can constrain a WHERE clause that limits the range of rows that can be returned from the view. However, this does not restrict the changes that can be made through the view. To restrict the changes that can be made, you use the WITH CHECK OPTION clause when defining the view.
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Lesson 4: Designing Database Views
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To understand the CHECK OPTION, a brief example is needed. A view could be defined for customers with a credit line greater than 1,000. A new customer could be inserted into the table with a credit line of 500 and not cause an error. However, this could cause confusion because although the insert was successful, the view would never be able to display the data, and a user might think that data had been lost. To restrict the changes that can be made so that the data is always visible through the view, you would define the view using the WITH CHECK OPTION clause. If the view were defined with the WITH CHECK OPTION clause, the insert of the customer with a credit line of 1,000 or less would cause an error to be returned. You can also create triggers on a view. Triggers created on views are a specialized case of a trigger called an INSTEAD OF trigger. INSTEAD OF triggers operate exactly as you might expect. Instead of the operation against the view being performed, the trig ger is executed to perform alternative operations.
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Creating an Indexed View
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A regular view is just a SELECT statement that is stored within SQL Server. When you reference it in a SELECT statement, the Query Optimizer substitutes the defini tion of the view into the query before executing the SELECT statement. So the only advantage a regular view provides is as a standardized abstraction for tables within the database. Any joins and aggregations need to be computed, and other operations still need to be performed at the time when the query is executed. Indexed views provide a way to precalculate the result set a view would return. This becomes useful when the cost to constantly execute the query far outweighs the cost required to maintain the results of the select statement in a view as data is modified. In theory, creating an indexed view is simply a process of creating a view and then creating a clustered index on the view. In practice, the process is not quite so straightforward. To create an indexed view, there are many restrictions on the way the base tables for the view are created. The view then has additional restrictions. Finally, the index has even more restrictions.
12
Designing the Physical Database
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Restrictions for creating an indexed view
For details on all the requirements and restrictions for creating an indexed view, please refer to the Books Online article, Creating Indexed Views.
The purpose for all the restrictions is to ensure that a consistent calculation can be made. An indexed view, also referred to as a materialized view, causes the select state ment in the view definition to be executed. A clustered index is built on the results, and then the data and index are permanently stored within the database. As data is changed on the base table(s), these changes are propagated to the indexed view. If the result of the view could change from one execution to another or could change if dif ferent query options were set, the entire set of data that was calculated and stored would be invalidated. Therefore, all the operators or functions that can cause varying results are disallowed. Some examples of these restrictions are as follows:
The select statement cannot reference other views. All functions must be deterministic; for example getdate() is not allowed because every time it is executed, a different result is returned. AVG, MIN, MAX, and STDEV are not allowed.
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