8.2 Modifying Views

Modifying Views

  • Discusses the circumstances under which it is possible to execute an insertion, deletion, or update to a view.
  • Introduces the concept of updatable views, where modifications to the view can be translated into modifications on a single base table.
  • Explains that for more complex views, ‘instead-of’ triggers can be used to define custom logic for handling view modifications.
#1

read: Section 8.2: Modifying Views (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Discusses the circumstances under which it is possible to execute an insertion, deletion, or update to a view.
- Introduces the concept of *updatable views*, where modifications to the view can be translated into modifications on a single base table.
- Explains that for more complex views, 'instead-of' triggers can be used to define custom logic for handling view modifications.

Modifying a view—that is, executing an insertion, deletion, or update—is not a straightforward operation. Since a view is a virtual relation defined by a query, it does not store data directly. The database system must translate any modification to the view into an equivalent modification on the underlying base tables. This translation is only possible under specific, limited circumstances.

  • Updatable Views

    In certain cases, a view is simple enough that modifications can be unambiguously translated to its underlying base table. Such views are termed updatable. SQL provides a formal definition for when a view can be modified.

    Definition: Updatable View
    A view is considered updatable if it satisfies the following conditions, among others:
    1. The FROM clause contains only one relation, which may itself be an updatable view.
    2. The SELECT clause does not contain the keyword DISTINCT.
    3. The WHERE clause must not contain a subquery that refers to the relation in the FROM clause.
    4. The attribute list in the SELECT clause must contain enough attributes to insert a new tuple into the base table. Specifically, any attribute of the base table that is not in the SELECT list must be nullable or have a default value.

  • Translating Modifications

    For an updatable view, modifications are passed through to the single underlying base relation.

    • An INSERT into the view becomes an INSERT into the base table. Attributes not present in the view are set to NULL or their default values. This can lead to the anomaly where the newly inserted tuple does not satisfy the view’s WHERE clause and is thus not visible through the view.
    • A DELETE from the view becomes a DELETE on the base table. The WHERE clause of the view is combined with the WHERE clause of the DELETE statement to ensure only tuples visible in the view are affected.
    • An UPDATE on the view becomes an UPDATE on the base table, similarly restricted by the view’s WHERE clause.

  • Instead-Of Triggers

    For views that are not updatable, such as those involving joins of multiple tables, the database system cannot determine how to propagate a modification. For instance, inserting a tuple into a view based on a join would require inserting corresponding tuples into multiple base tables, an ambiguous operation.

    To address this, SQL provides INSTEAD OF triggers. These triggers allow a programmer to define a specific procedure that executes in place of a modification attempt on the view. This provides a mechanism for handling updates on complex views by explicitly stating how insertions, deletions, or updates should be translated into operations on the underlying base tables.

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View Removal

  • Explains that a view’s definition can be deleted using the DROP VIEW statement.
  • Clarifies that dropping a view does not affect the tuples of the underlying base relation(s).
  • Contrasts DROP VIEW with DROP TABLE, noting that dropping a base table would render any views that depend on it unusable.
#2

read: Section 8.2.1: View Removal (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Explains that a view's definition can be deleted using the `DROP VIEW` statement.
- Clarifies that dropping a view does not affect the tuples of the underlying base relation(s).
- Contrasts `DROP VIEW` with `DROP TABLE`, noting that dropping a base table would render any views that depend on it unusable.

An extreme modification to a view is to delete its definition entirely. This action can be performed on any view, irrespective of whether it is updatable.

Definition: View Removal

The DROP VIEW statement is a Data Definition Language (DDL) command that removes a view’s definition from the database schema.

DROP VIEW view_name;

  • The DROP VIEW statement deletes the definition of a view.

    This action removes the stored query that constitutes the view. After a view is dropped, it can no longer be referenced in queries or other database commands. For example, DROP VIEW ParamountMovies; deletes the definition of the ParamountMovies view.

  • Dropping a view does not affect the data in the underlying base tables.

    The tuples of the base relations from which the view is derived remain entirely unaffected. The view is merely a logical construct; its removal does not alter the physical data stored in the tables.

  • Dropping a base table renders dependent views unusable.

    In contrast, if a base table is removed with a DROP TABLE statement, any view that depends on that table becomes invalid. A query attempting to use such a view will fail because its underlying data source no longer exists. For instance, DROP TABLE Movies would make the ParamountMovies view unusable.

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Updatable Views

  • Provides the formal definition from SQL for when modifications to a view are permitted.
  • States the primary condition for an updatable view: it must be defined by selecting attributes from a single relation, R.
  • Lists important technical points for a view to be updatable: the WHERE clause must not involve R in a subquery, the FROM clause must only contain one occurrence of R, and the SELECT list must include enough attributes to fill other attributes with NULL or default values on insertion.
  • Describes how INSERT, DELETE, and UPDATE operations on an updatable view are translated to the underlying base table.
#3

read: Section 8.2.2: Updatable Views (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Provides the formal definition from SQL for when modifications to a view are permitted.
- States the primary condition for an updatable view: it must be defined by selecting attributes from a single relation, R.
- Lists important technical points for a view to be updatable: the `WHERE` clause must not involve R in a subquery, the `FROM` clause must only contain one occurrence of R, and the `SELECT` list must include enough attributes to fill other attributes with `NULL` or default values on insertion.
- Describes how `INSERT`, `DELETE`, and `UPDATE` operations on an updatable view are translated to the underlying base table.
Definition: Updatable View

SQL permits modifications on a view if it is defined by a query that selects attributes from a single base relation, \(R\), which may itself be an updatable view. Such a view is termed “updatable.” The query must not use SELECT DISTINCT.

  • Primary Condition

    The view must be defined by selecting attributes from exactly one relation, denoted as \(R\). This relation \(R\) may be a base table or another updatable view.

  • Technical Points for Updatability

    SQL imposes several technical constraints on the query defining the view for it to be considered updatable.

    • The FROM clause must contain only a single occurrence of the relation \(R\). No other relations are permitted.
    • The WHERE clause must not contain a subquery that involves the relation \(R\).
    • The attribute list in the SELECT clause must be sufficient for insertion operations. Specifically, it must not project out any attribute of \(R\) that is declared NOT NULL and lacks a default value. This ensures that a complete tuple can be formed for the base table.

  • Insertion (INSERT)

    An INSERT statement on an updatable view is translated into an INSERT on the underlying base relation \(R\).

    Values are provided only for the attributes present in the view’s SELECT clause. All other attributes in the base table \(R\) are assigned NULL or their specified default values. It is possible for an inserted tuple to not satisfy the view’s WHERE clause, and thus not be visible in the view after insertion.

  • Deletion (DELETE)

    A DELETE statement on an updatable view is translated into a DELETE on the underlying base relation \(R\).

    To ensure only tuples visible in the view are deleted, the condition from the view’s WHERE clause is conjoined (using AND) with the condition in the DELETE statement’s WHERE clause.

  • Update (UPDATE)

    An UPDATE statement on an updatable view is translated into an UPDATE on the underlying base relation \(R\).

    Similar to deletion, the condition from the view’s WHERE clause is conjoined (using AND) with the condition in the UPDATE statement’s WHERE clause. This restricts the update operation to only those tuples that are visible through the view.

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Instead-Of Triggers on Views

  • Explains that triggers on views can use INSTEAD OF in place of BEFORE or AFTER.
  • Describes how an INSTEAD OF trigger intercepts an attempt to modify a view and performs the trigger’s specified action instead of the original event.
  • This mechanism allows database designers to implement custom logic for modifications on views that are not natively updatable.
#4

read: Section 8.2.3: Instead-Of Triggers on Views (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Explains that triggers on views can use `INSTEAD OF` in place of `BEFORE` or `AFTER`.
- Describes how an `INSTEAD OF` trigger intercepts an attempt to modify a view and performs the trigger's specified action instead of the original event.
- This mechanism allows database designers to implement custom logic for modifications on views that are not natively updatable.

When a trigger is defined on a view, one may use INSTEAD OF in place of BEFORE or AFTER. This construct provides a powerful mechanism for controlling the behavior of data manipulation language (DML) operations on views, particularly those that are not inherently updatable.

Definition: Instead-Of Trigger

An INSTEAD OF trigger is a trigger defined on a view that specifies an action to be executed in place of the DML operation (e.g., INSERT, UPDATE, or DELETE) that fired it. The original DML operation against the view is not performed; instead, the logic defined within the trigger is executed.

  • Operation Interception

    When a DML event awakens the trigger, the INSTEAD OF clause causes the system to bypass the event itself. The action of the trigger is performed as a substitute for the attempted modification on the view.

  • Enabling Updates on Complex Views

    This mechanism allows the database designer to implement custom logic for view modifications. It is especially useful for views that are not updatable according to standard SQL rules, such as those involving joins, aggregation, or unions. The trigger can translate a single operation on the view into the correct sequence of operations on the underlying base tables.

  • Custom Behavior

    The programmer can force any desired interpretation of a view modification. For instance, an INSERT on a view can be programmed to perform insertions into multiple base tables, assign default values, or execute complex validation logic before modifying the underlying data.

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