Inserting, updating, and deleting views

In SQL, views are generally intended for data retrieval purposes. However, under certain conditions, you can also use INSERT, UPDATE, and DELETE statements on views, but there are significant restrictions.

INSERTING

When inserting data using a view, the data types in the INSERT statement must match the data types in the view's base table. Also, you can only insert data into a view that is based on a single table, and not all columns from the base table are required to be in the view.

Here's an example:

CREATE VIEW SimpleCustomerView AS
SELECT CustomerID, CustomerName
FROM Customers;

INSERT INTO SimpleCustomerView (CustomerID, CustomerName)
VALUES (101, 'New Customer');

UPDATING

Like with INSERT, you can only perform an UPDATE operation on a view if it is based on a single table. The fields you're updating must be directly linked to that underlying table.

Example:

UPDATE SimpleCustomerView
SET CustomerName = 'Updated Customer Name'
WHERE CustomerID = 101;

DELETING

You can also delete from a view with similar restrictions. Any row that you delete from a view is also deleted from the base table.

Example:

DELETE FROM SimpleCustomerView
WHERE CustomerID = 101;

It's important to note the following restrictions:

If the view was created using a complex query, involving multiple tables, JOINs, or certain kinds of subqueries, you cannot insert, update, or delete rows in the view.
If the view contains calculated or aggregated fields (like SUM, AVG), you cannot insert, update, or delete rows.
If the view was created with a GROUP BY clause, you cannot insert, update, or delete rows.
If the view was created with the DISTINCT keyword, you cannot insert, update, or delete rows.
Not all database systems support inserting, updating, and deleting data through views. Check the documentation for your specific system.

Remember, any changes made to the data in the view are made to its base table. If the base table is read-only, the view will also be read-only.

In some cases, instead of directly inserting, updating, or deleting on the view, you might want to create INSTEAD OF triggers on the view to provide custom behavior for these operations. This is particularly useful for views based on multiple tables.