Friday, June 3, 2011

Updatable Views

You can modify the data of an underlying base table through a view, as long as the following conditions are true:
  • Any modifications, including UPDATE, INSERT, and DELETE statements, must reference columns from only one base table.
  • The columns being modified in the view must directly reference the underlying data in the table columns. The columns cannot be derived in any other way, such as through the following:
    • An aggregate function: AVG, COUNT, SUM, MIN, MAX, GROUPING, STDEV, STDEVP, VAR, and VARP.
    • A computation. The column cannot be computed from an expression that uses other columns. Columns that are formed by using the set operators UNION, UNION ALL, CROSSJOIN, EXCEPT, and INTERSECT amount to a computation and are also not updatable.
  • The columns being modified are not affected by GROUP BY, HAVING, or DISTINCT clauses.
  • TOP is not used anywhere in the select_statement of the view together with the WITH CHECK OPTION clause.
The previous restrictions apply to any subqueries in the FROM clause of the view, just as they apply to the view itself. Generally, the Database Engine must be able to unambiguously trace modifications from the view definition to one base table. For more information, see Modifying Data Through a View.
If the previous restrictions prevent you from modifying data directly through a view, consider the following options:
  • INSTEAD OF Triggers
    INSTEAD OF triggers can be created on a view to make a view updatable. The INSTEAD OF trigger is executed instead of the data modification statement on which the trigger is defined. This trigger lets the user specify the set of actions that must happen to process the data modification statement. Therefore, if an INSTEAD OF trigger exists for a view on a specific data modification statement (INSERT, UPDATE, or DELETE), the corresponding view is updatable through that statement. For more information about INSTEAD OF triggers, see Designing INSTEAD OF Triggers.
  • Partitioned Views
    If the view is a partitioned view, the view is updatable, subject to certain restrictions. When it is needed, the Database Engine distinguishes local partitioned views as the views in which all participating tables and the view are on the same instance of SQL Server, and distributed partitioned views as the views in which at least one of the tables in the view resides on a different or remote server.
    For more information about partitioned views, see Creating Partitioned Views.

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