3.8

A view is a virtual table based on the result set of a SELECT statement.
The CREATE VIEW statement is used to create a view table:
- Specifies the view name.
- The view query defines the data within the view.
- Can optionally specify column names; if not specified, they are the same as in the result of the view query.

View data is not stored; it appears dynamically in SQL statements.
A materialized view:
- Stores its data permanently.
- Must be refreshed whenever the underlying base tables change.
Base tables serve as the original source tables and are created as tables (not as views).

Views save complex or optimized queries for improved database performance.
The performance of a user’s query on a view matches that of the corresponding direct query on base tables.
Views can reference other views.
Views can hide specific rows and columns from users for simplified access.

Inserting and updating records can be problematic:
- A primary key in the base table that does not appear in a view can create issues; inserting into the view might generate a null primary key value, which is not allowed due to primary key constraints.
- Aggregated values on the view cannot be easily converted into base table values, thus restricting updates or inserts involving aggregation.
- Join views may cause additional complications, as the effect of deletion in join views is undefined and therefore not allowed.

The WITH CHECK option rejects inserts and updates that do not satisfy the view query's WHERE clause.
When a violation occurs, the database generates an error message indicating the restriction or violation.