SQL Views and Types of Views

In SQL, a view is a virtual table that is based on the result of a SELECT statement. Views allow you to simplify complex queries by creating a pre-defined query that can be used in other queries. Views also provide a level of abstraction, as they can hide the complexity of the underlying data model from the user.

Here is an example of how to create a view in SQL:

CREATE VIEW EmployeeData AS
SELECT EmployeeID, FirstName, LastName, DepartmentName, Salary
FROM Employees
JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

In this example, we are creating a view called EmployeeData. The view is based on a SELECT statement that retrieves data from two tables, Employees and Departments. The view selects the EmployeeID, FirstName, LastName, DepartmentName, and Salary columns from the two tables and joins them using the DepartmentID column.

Once the view has been created, it can be used like a regular table in other queries. For example:

SELECT *
FROM EmployeeData
WHERE Salary > 50000;

This query retrieves all columns from the EmployeeData view where the Salary is greater than 50000.

Views can also be used to simplify complex joins and calculations. For example, the following view calculates the total sales amount for each salesperson:

CREATE VIEW SalespersonTotalSales AS
SELECT SalespersonID, SUM(Amount) as TotalSales
FROM Sales
GROUP BY SalespersonID;
 

In this example, we are creating a view called SalespersonTotalSales. The view is based on a SELECT statement that calculates the total sales amount for each salesperson in the Sales table. The view selects the SalespersonID column and uses the SUM function to calculate the TotalSales column.

Once the view has been created, it can be used in other queries to simplify the calculation of the total sales amount for each salesperson. For example:

SELECT Salespeople.FirstName, Salespeople.LastName, SalespersonTotalSales.TotalSales
FROM Salespeople
JOIN SalespersonTotalSales
ON Salespeople.SalespersonID = SalespersonTotalSales.SalespersonID;
 

This query retrieves the FirstName and LastName columns from the Salespeople table and the TotalSales column from the SalespersonTotalSales view, which calculates the total sales amount for each salesperson. The query joins the two tables using the SalespersonID column.

Types of SQL Views 

There are two types of views in SQL: simple views and complex views.

Simple Views: A simple view is a view that is based on a single table. It selects a subset of the columns and rows of the table, and can also rename the columns or perform basic calculations. Simple views are easy to create and can be used to simplify complex queries or to provide a more meaningful view of the data.

Example of simple view:

CREATE VIEW Customers_View AS
SELECT CustomerID, CompanyName, ContactName, City
FROM Customers
WHERE Country = 'USA';
 

This creates a simple view called Customers_View based on the Customers table, selecting only the columns CustomerID, CompanyName, ContactName, and City for customers located in the USA. 

Complex Views: A complex view is a view that is based on multiple tables. It can perform more advanced operations, such as grouping, joining, and aggregating data from multiple tables. Complex views are more difficult to create and maintain than simple views, but they can be very powerful in providing a simplified and meaningful view of complex data.

Example of complex view:

CREATE VIEW Sales_View AS
SELECT Customers.CompanyName, Employees.FirstName, Employees.LastName, SUM(OrderDetails.Quantity * OrderDetails.UnitPrice) AS TotalSales
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID
INNER JOIN OrderDetails ON Orders.OrderID = OrderDetails.OrderID
GROUP BY Customers.CompanyName, Employees.FirstName, Employees.LastName;
 

This creates a complex view called Sales_View that calculates the total sales for each customer and employee combination based on the Orders, Customers, Employees, and OrderDetails tables. It uses inner joins to combine the data from the four tables and the SUM function to calculate the total sales. Finally, it groups the data by CompanyName, FirstName, and LastName.

In summary, simple views are based on a single table and perform basic operations, while complex views are based on multiple tables and perform more advanced operations. Both types of views can be used to simplify complex queries and provide a more meaningful view of the data.