Common Table Expression (CTE) in Sql Server

Common Table Expressions, or CTEs, can be thought of as temporary views that are created in memory for a single execution scope. They allow you to simplify complex joins and subqueries by defining a named result set that exists only until the next query runs. Whether you are using Recursive or Non-Recursive CTEs, understanding their limitations—such as being unindexable and requiring immediate execution—is key to writing efficient SQL code.

• We can take CTE as tempory view (can be thought as tempory result set) that create in memory

• Its scope valid until next query run

• CTE need to use just after it declaration  

• There are two type Recursive and Nonrecursive CTE

• Can define multiple CTE

• CTE are unindexable ( it cau use exist index in referenced object)

• CTE can't have constrain

• ORDER BY clause can't be used in the CTE_Query_Definition , execpt when use TOP keyword

Simple Example.

ex 1

WITH CTE_CUSTOMER_EX2
(         [CustomerID]
          ,[CompanyName]
  ,[ContactName]
  ,[ContactTitle]
  ,[Address]
  ,[City]
)
AS
(
SELECT [CustomerID]
  ,[CompanyName]
  ,[ContactName]
  ,[ContactTitle]
  ,[Address]
  ,[City]

  FROM .[dbo].[Customers]
)


SELECT * FROM CTE_CUSTOMER_EX2

ex 2

;WITH CTE_CUSTOMER_EX1
AS
(
SELECT [CustomerID]
 ,[CompanyName]
 ,[ContactName]
 FROM .[dbo].[Customers]

)
SELECT * FROM CTE_CUSTOMER_EX1

Note
If there are query before CTE define , need to use Semicolon teminator before declare CTE


Multiple CTE

  With CTE_M1 as 
(
-select statement
) 
     , CTE_M2 as 
(
-select statemet

)

you can find some more detail and examples using this Link and Link2