In Microsoft SQL Server, views are database objects that represent virtual tables created by defining a SQL query. Views provide a way to simplify complex queries, encapsulate business logic, and control access to data by presenting a subset of a table or joining multiple tables. Here’s how to create, alter, and drop views in SQL Server:

1. Creating a View:

To create a view, you use the CREATE VIEW statement. The view definition is a SQL query that specifies the columns and rows to include in the virtual table. Views are typically created in the context of a specific database.

Syntax for Creating a View:

CREATE VIEW view_name AS SELECT column1, column2, ... FROM table_name WHERE condition;

Example of Creating a View:

Suppose you have a database with a table named Orders, and you want to create a view that displays only the orders with a status of ‘Shipped’:

CREATE VIEW ShippedOrders AS SELECT OrderID, CustomerID, OrderDate FROM Orders WHERE OrderStatus = 'Shipped';

After creating this view, you can query it like a regular table to retrieve shipped orders.

2. Altering a View:

You can modify the definition of an existing view using the ALTER VIEW statement. This allows you to change the underlying query or the columns included in the view.

Syntax for Altering a View:

ALTER VIEW view_name AS SELECT new_column1, new_column2, ... FROM new_table_name WHERE new_condition;

Example of Altering a View:

Suppose you want to modify the ShippedOrders view to include additional columns and change the filtering condition:

ALTER VIEW ShippedOrders AS SELECT OrderID, CustomerID, OrderDate, ShipMethod FROM Orders WHERE OrderStatus = 'Shipped' AND ShipDate IS NOT NULL;

This altered view now includes the ShipMethod column and filters for orders that have been shipped and have a non-null ship date.

3. Dropping a View:

To remove a view from the database, you use the DROP VIEW statement.

Syntax for Dropping a View:

DROP VIEW view_name;

Example of Dropping a View:

To delete the ShippedOrders view:

DROP VIEW ShippedOrders;

Considerations:

• Views are read-only by default, meaning you can query data through views but cannot perform data modifications (insert, update, delete) directly on them. However, you can create updatable views by defining appropriate triggers or using the INSTEAD OF trigger.
• Views are helpful for abstracting complex queries and presenting a simplified view of data to application developers or end-users.
• When creating or altering views, ensure that the underlying table structure and data are compatible with the view definition.

Creating, altering, and dropping views in SQL Server is a fundamental aspect of database design and management. Views enhance data security, simplify query construction, and promote data abstraction and separation of concerns within your database schema.