Difference between Simple and Complex View in SQL

In SQL, a view is a virtual table based on the result set of an SQL statement. It contains rows and columns, just like a real table, but it doesn't store data. Instead, the query defining the view references data from one or more tables.

There are mainly two types of views: Simple Views and Complex Views. Here are the primary differences between them:

Simple View:
- Single Table: A simple view is derived from a single table. It doesn't involve multiple tables.
- No Functions: It does not contain functions, groups, or aggregate calculations.
- DML Operations: You can often perform INSERT, UPDATE, and DELETE operations through a simple view (assuming there are no constraints to prevent these operations on the underlying table).
- No Joins: It doesn't involve joins.
Example:
```
CREATE VIEW SimpleView AS
SELECT first_name, last_name
FROM employees;
```
Complex View:
- Multiple Tables: A complex view can be derived from more than one table.
- Contains Functions: It can contain functions, groups, or aggregate calculations.
- DML Operations: Typically, you cannot perform all DML operations (INSERT, UPDATE, DELETE) through a complex view. However, this might depend on the complexity of the view and the database system.
- Has Joins: It often involves joins, unions, or subqueries.
Example:
```
CREATE VIEW ComplexView AS
SELECT e.first_name, e.last_name, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.department_id
WHERE e.salary > 50000;
```

While these distinctions generally hold true, the exact capabilities and restrictions can vary depending on the database system. For instance, some advanced RDBMSs have capabilities that allow for certain DML operations on more complex views given specific conditions. Always refer to the documentation of the specific SQL system you're working with for precise definitions and capabilities.

Characteristics of Simple Views in SQL:
- Simple views are based on a single table.
- They consist of a SELECT statement that retrieves data from one table.
- Simple views are easy to create and understand.
```
-- Simple View
CREATE VIEW EmployeeView AS
SELECT EmployeeID, FirstName, LastName
FROM Employees;
```

Defining and Creating Simple Views in SQL:

CREATE VIEW ProductView AS
SELECT ProductID, ProductName, Price
FROM Products
WHERE Price > 100;

Use Cases for Complex Views in SQL Queries:
- When data retrieval requires information from multiple related tables.
- Aggregating and summarizing data using GROUP BY and aggregate functions.
- Incorporating business logic or complex calculations within the view.
```
-- Complex View with JOIN and Aggregation
CREATE VIEW SalesSummary AS
SELECT c.CustomerID, c.CustomerName, SUM(s.Amount) AS TotalSales
FROM Customers c
JOIN Sales s ON c.CustomerID = s.CustomerID
GROUP BY c.CustomerID, c.CustomerName;
```

Creating and Managing Complex Views in a Database:

-- Complex View
CREATE VIEW EmployeeDepartmentView AS
SELECT e.EmployeeID, e.FirstName, e.LastName, d.DepartmentName
FROM Employees e
JOIN Departments d ON e.DepartmentID = d.DepartmentID;