Read Only View

A read-only view in Oracle is a type of view that allows you to retrieve data but prevents modifications (such as INSERT, UPDATE, or DELETE) to the underlying data through the view. Essentially, it is a view that can only be queried, not altered. This behavior is useful when you want to allow users to view certain data without allowing them to change it.

1. What is a Read-Only View?

A read-only view is a view that restricts the user from performing data-modifying operations like INSERT, UPDATE, or DELETE through the view. The view can still be queried using SELECT to retrieve data, but no changes can be made to the data via this view.

For example, a view can aggregate data or filter rows based on specific criteria, and if certain conditions are met, it will be marked as read-only.

2. How is a View Made Read-Only?

A view becomes read-only in the following scenarios:

• Simple Views: Views that are based on a single table or a simple query without JOINs, GROUP BY, or aggregation functions are typically updatable, but if they involve any conditions or complexity that make them non-updatable, they become read-only.
• Joins, Aggregations, and Grouping: Views that involve multiple tables (JOIN), group data (GROUP BY), or aggregate functions (SUM(), COUNT(), AVG(), etc.) are often read-only. Oracle cannot track how these views relate to the underlying data for modification.
• Using DISTINCT: If a view uses the DISTINCT keyword to eliminate duplicates, it may become read-only, as Oracle can't guarantee that data changes will be consistent with the view's logic.

3. Characteristics of Read-Only Views

• Cannot be Updated: You cannot update data through the view if it involves certain operations (e.g., multiple tables in a join or aggregation).
• Cannot be Inserted or Deleted: Like updates, inserts and deletes are not allowed if the view involves complex operations like joins, unions, or aggregation.
• Data Retrieval Only: You can only use a SELECT statement to retrieve data from the view, not to modify it.
• Aggregation: Views that perform aggregation functions (like SUM(), COUNT(), MAX(), etc.) are typically read-only because the results are computed dynamically and cannot be directly updated.

4. Why Use Read-Only Views?

Read-only views are often used to:

• Restrict Modifications: Prevent users from modifying critical data while still allowing them to query and analyze it.
• Enhance Security: By providing a limited, read-only interface to the data, you can ensure that only certain users or applications can modify the data.
• Data Abstraction: Create views that aggregate, filter, or transform data for reporting or analytical purposes without altering the underlying tables.
• Simplify Queries: Complex queries with multiple joins or aggregations can be encapsulated in a read-only view, making it easier for users to access summarized data without needing to understand the underlying complexity.

5. Example of a Read-Only View

Consider a table employees with the following schema:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    first_name VARCHAR2(50),

    last_name VARCHAR2(50),

    salary NUMBER,

    department_id NUMBER

);

A view that simply filters employees by salary can be created as follows:

CREATE VIEW high_salary_employees AS

SELECT employee_id, first_name, last_name, salary

FROM employees

WHERE salary > 50000;

This view is read-only in its current state because:

• It only filters the data by salary > 50000.
• The underlying table (employees) is the source of the data.
• Users can query the view using SELECT, but cannot INSERT, UPDATE, or DELETE data via this view.

6. When Can a View Be Updatable (Not Read-Only)?

A view will not be read-only if it meets the following conditions:

• Single Table View: The view is based on a single table, and no complex joins, aggregations, or transformations are used.
• No Aggregate Functions: If the view does not use SUM(), AVG(), COUNT(), or other aggregation functions, and it does not contain a GROUP BY clause.
• No DISTINCT: The use of the DISTINCT keyword can make a view read-only because Oracle cannot track how duplicates are created or eliminated.
• Simple WHERE Clauses: If the view includes simple conditions in the WHERE clause, it remains updatable.
• No Complex Joins: A view that uses multiple JOIN operations between tables is more likely to be read-only.

7. Example of an Updatable View

CREATE VIEW updatable_view AS

SELECT employee_id, first_name, last_name, salary

FROM employees;

This view is updatable because:

• It involves only one table (employees).
• It doesn't involve any aggregation, joins, or distinct operations.
• You can perform INSERT, UPDATE, and DELETE operations on the underlying employees table through the view.

8. Conditions That Make a View Read-Only

Several conditions can make a view read-only in Oracle:

• The view includes JOIN operations between multiple tables.
• The view includes aggregate functions (e.g., SUM(), COUNT(), MAX(), AVG()).
• The view includes GROUP BY or HAVING clauses.
• The view includes DISTINCT to eliminate duplicates.
• The view involves subqueries in the SELECT, FROM, or WHERE clauses.

9. How to Check if a View is Read-Only

To check whether a view is read-only, you can try to perform an insert, update, or delete operation on the view. If Oracle raises an error, then the view is read-only.

You can also check the UPDATABLE property of the view by using DESCRIBE or querying the ALL_VIEWS view in Oracle. However, the most definitive way to check is by attempting a data modification operation.

10. Can I Make a Read-Only View Writable Again?

Yes, you can make a read-only view writable again by:

• Simplifying the View: Remove any complex operations like joins, aggregation, DISTINCT, or GROUP BY clauses.
• Rewriting the View: Modify the view definition to remove the factors that make it read-only.

For example, if the high_salary_employees view above becomes too complex or includes aggregation, you could rewrite it as:

CREATE VIEW updatable_high_salary_employees AS

SELECT employee_id, first_name, last_name, salary

FROM employees

WHERE salary > 50000 AND department_id = 10;

If the view is based on a single table and contains no aggregation or joins, it can be made updatable.

11. Benefits of Using Read-Only Views

• Data Protection: Prevents unauthorized or accidental modification of critical data.
• Centralized Logic: Simplifies complex data transformations by encapsulating them in a view.
• Security: Limits access to the underlying data while allowing users to access the results of complex queries.
• Simplified User Access: Provides users with a simplified interface to access data without exposing the underlying schema or business logic.

12. Dropping a Read-Only View

A read-only view can be dropped like any other view using the DROP VIEW statement:

DROP VIEW high_salary_employees;

This will remove the view, but the underlying data in the employees table remains unchanged.

 

Conclusion

A read-only view in Oracle is an important tool for controlling data access, ensuring security, and maintaining data integrity by preventing unauthorized changes. By carefully designing views with appropriate logic and restrictions, you can allow users to retrieve information without putting your data at risk.