Introduction to Object Views

are not usually updatable. To overcome this, Oracle allows INSTEAD OF triggers to be created on these columns.

The INSTEAD OF trigger defined on a nested table column of a view is fired when the column is modified. If the entire collection is replaced by an update of the parent row, then the INSTEAD OF trigger on the nested table column is not fired.

See Also: Oracle9i Application Developer’s Guide - Fundamentals for a purchase order/line item example that uses an INSTEAD OF trigger on a nested table column

View Hierarchies

An object view can be created as a subview of another object view. The type of the superview must be the immediate supertype of the type of the object view being created. Thus, you can build an object view hierarchy which has a one-to-one correspondence to the type hierarchy. This does not imply that every view hierarchy must span the entire corresponding type hierarchy. The view hierarchy can be rooted at any subtype of the type hierarchy. Furthermore, it does not have to encompass the entire subhierarchy.

Figure 13–2 Multiple View Hierarchies

VH1 VH2

By default, the rows of an object view in a view hierarchy include all the rows of all its subviews (direct and indirect) projected over the columns of the given view.

Only one object view can be created as a subview of a given view corresponding to the given subtype; that is, the same view cannot participate in many different view hierarchies. An object view can be created as a subview of only one superview; multiple inheritance is not supported.

The subview inherits the object identifier (OID) from its superview and cannot be explicitly specified in any subview.

Object Datatypes and Object Views 13-27

Introduction to Object Views

13-28 Oracle9i Database Concepts

Part V

Data Access

Part V describes how to use transactions consisting of SQL statements to access data in an Oracle database. It also describes the procedural language constructs that provide additional functionality for data access.

Part V contains the following chapters: