CTE: Common Table Expressions

CTE is a useful tool to generate temporary tables used only within a query, which helps to simplify complex queries.
CTE must be followed by a single SELECT, INSERT, UPDATE or DELETE statement that references CTE columns.
WITH clause can include one or more CTE separated by using "," (commas)
Enables users to write and maintain complex queries more easily with good readability and simplification.

-- Syntax:
WITH CTE_Name AS (CTE Query...);

-- Example 1: Retrieve a snapshot from a table.
WITH CTE1 
AS (SELECT * FROM Employee LIMIT 10)
SELECT * FROM CTE1;

-- Example 2: Retrieve Maximum Average Salary from Employee Table.
WITH CTE2 
AS (SELECT Dept_ID, AVG(Salary) AS AVG_Salary FROM Employee GROUP BY Dept_ID)
SELECT MAX(AVG_Salary) FROM CTE2;