Views

A view in SQL is a virtual or logical table based on the result-set of an SQL statement. It contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.

A view can be used for various purposes such as:

To focus on specific data by saving a particular query with specific filters
To simplify complex queries by encapsulating parts of the query
To protect data by exposing only necessary data and hiding details in underlying table structure
To provide a consistent, unchanged view of data even if the underlying data changes (in terms of schema)

You can create a view in SQL using the CREATE VIEW statement. Here is the basic syntax:

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

Here is an example:

CREATE VIEW Top_Customers AS
SELECT CustomerName, ContactName 
FROM Customers
WHERE Orders > 50;

In this example, the Top_Customers view would show the CustomerName and ContactName for all customers who have placed more than 50 orders.

Once the view is created, you can query it as you would query an actual table. For example:

SELECT * FROM Top_Customers;

It's important to note that views are not physically materialized anywhere in the database. They are computed on-the-fly when accessed and the data comes directly from the tables that the view is based on. As a result, changes to the data in those tables will be reflected in the view.

A view in SQL provides several benefits:

Simplification of Complex Queries: Views can be used to encapsulate complex queries with joins, subqueries, aggregations, and calculations into a single simple query, making data access simpler and more intuitive.
Security: Views can provide a security mechanism to protect certain data in your database. By creating a view, you can control the level of access that different users have to the underlying database tables. Users can be granted permission to access the views, without exposing the entire table to them.
Consistent and Simplified Interface: Views present a consistent, unchanging image of the structure of the database, even if the underlying tables are split, restructured, or renamed. Therefore, applications that use views are insulated from changes in the database schema.
Data Abstraction: Views can present a different structure of the underlying schema data. It can combine several tables into one virtual table, or extract certain data from a single table. This can be particularly useful in large databases where there are multiple tables.
Data Integrity: If data insertion, update, and deletion are performed through a view, the DBMS can automatically check these operations to ensure that all defined view constraints are satisfied.
Calculation Simplification: Views can be used to encapsulate complex calculations. For instance, if you're consistently running a query that calculates total sales, you could create a view that handles this calculation, simplifying future queries.
Performance: Depending on the database system, some DBMSs can store the result of a view in memory, improving query performance. However, this is not universal and depends on the specific DBMS and configuration.

Remember, while views provide these advantages, they also have their limitations. For instance, in many DBMSs, views are read-only, so you can't use them to insert, update, or delete data. Additionally, because views are based on underlying tables, they can't be used to permanently store data or persist data independently of those tables.