In Oracle, is it possible to INSERT or UPDATE a record through a view?

Question

In Oracle, is it possible to INSERT or UPDATE a record (a row) through a view?

Answer 1

Views in Oracle may be updateable under specific conditions. It can be tricky, and usually is not advisable.

From the Oracle 10g SQL Reference:

Notes on Updatable Views

An updatable view is one you can use to insert, update, or delete base table rows. You can create a view to be inherently updatable, or you can create an INSTEAD OF trigger on any view to make it updatable.

To learn whether and in what ways the columns of an inherently updatable view can be modified, query the USER_UPDATABLE_COLUMNS data dictionary view. The information displayed by this view is meaningful only for inherently updatable views. For a view to be inherently updatable, the following conditions must be met:

• Each column in the view must map to a column of a single table. For example, if a view column maps to the output of a TABLE clause (an unnested collection), then the view is not inherently updatable.
• The view must not contain any of the following constructs:
  • A set operator
  • a DISTINCT operator
  • An aggregate or analytic function
  • A GROUP BY, ORDER BY, MODEL, CONNECT BY, or START WITH clause
  • A collection expression in a SELECT list
  • A subquery in a SELECT list
  • A subquery designated WITH READ ONLY
  • Joins, with some exceptions, as documented in Oracle Database Administrator's Guide

In addition, if an inherently updatable view contains pseudocolumns or expressions, then you cannot update base table rows with an UPDATE statement that refers to any of these pseudocolumns or expressions.

If you want a join view to be updatable, then all of the following conditions must be true:

• The DML statement must affect only one table underlying the join.
• For an INSERT statement, the view must not be created WITH CHECK OPTION, and all columns into which values are inserted must come from a key-preserved table. A key-preserved table is one for which every primary key or unique key value in the base table is also unique in the join view.
• For an UPDATE statement, all columns updated must be extracted from a key-preserved table. If the view was created WITH CHECK OPTION, then join columns and columns taken from tables that are referenced more than once in the view must be shielded from UPDATE.
• For a DELETE statement, if the join results in more than one key-preserved table, then Oracle Database deletes from the first table named in the FROM clause, whether or not the view was created WITH CHECK OPTION.

Answer 2

Oracle has two different ways of making views updatable:-

1. The view is "key preserved" with respect to what you are trying to update. This means the primary key of the underlying table is in the view and the row appears only once in the view. This means Oracle can figure out exactly which underlying table row to update OR
2. You write an instead of trigger.

I would stay away from instead-of triggers and get your code to update the underlying tables directly rather than through the view.

Answer 3

There are two times when you can update a record through a view:

1. If the view has no joins or procedure calls and selects data from a single underlying table.
2. If the view has an INSTEAD OF INSERT trigger associated with the view.

Generally, you should not rely on being able to perform an insert to a view unless you have specifically written an INSTEAD OF trigger for it. Be aware, there are also INSTEAD OF UPDATE triggers that can be written as well to help perform updates.