What is Recursive CTE ?

                      Recursive is the process in which the query executes itself. It is used to get results based on the output of base query. We can use CTE as Recursive CTE (Common Table Expression).
  Consider following query

WITH Emp_CTE AS
(
SELECT EmployeeID, ContactID, LoginID, ManagerID, Title, BirthDate FROM HumanResources.Employee WHERE ManagerID IS NULL
UNION ALL
SELECT e.EmployeeID, e.ContactID, e.LoginID, e.ManagerID, e.Title,e.BirthDate FROM HumanResources.Employee e
INNER JOIN Emp_CTE ecte ON ecte.EmployeeID = e.ManagerID
)
SELECT *FROM Emp_CTE
GO

                           In the above example Emp_CTE is a Common Expression Table, the base record for the CTE is derived by the first sql query before UNION ALL. The result of the query gives you the EmployeeID which don’t have ManagerID.
                           Second query after UNION ALL is executed repeatedly to get results and it will continue until it returns no rows. For above e.g. Result will have EmployeeIDs which have ManagerID (ie, EmployeeID of the first result).  This is obtained by joining CTE result with Employee table on columns EmployeeID of CTE with ManagerID of table Employee.
                      This process is recursive and will continue till there is no ManagerID who doesn’t have EmployeeID