Recursive CTE Concept Confusion

Question

I am trying to understand the concepts of using CTE in my SQL code. I have gone through a number of online posts explaining the concept but I cannot grasp how it iterates to pre

Answer 1

So you want to understand a recursive CTE.

It's simple really.

First there's the seed query which gets the original records.

  SELECT EmployeeID, ContactID, LoginID, ManagerID, Title, BirthDate
  FROM HumanResources.Employee
  WHERE ManagerID IS NULL

In your case it's the employees without a manager.
Which would be the boss(es)

To demonstrate with a simplified example:

EmployeeID LoginID ManagerID Title 
---------- ------- --------- ------------
101        boss    NULL      The Boss

The second query looks for employees that have the previous record as a manager.

  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

Since it's a recursive CTE, the CTE uses itself in the second query.
You could see it as a loop, where it uses the previous records to get the next.

For the first iteration of that recursive loop you could get something like this:

 EmployeeID LoginID ManagerID Title 
---------- ------- --------- ------------
102        head1    101      Top Manager 1
103        head2    101      Top Manager 2

For the second iteration it would use the records from that first iteration to find the next.

 EmployeeID LoginID ManagerID Title 
---------- ------- --------- ------------

104        bob     102       Department Manager 1
105        hilda   102       Department Manager 2

108        john    103       Department Manager 4
109        jane    103       Department Manager 5

For the 3th iteration it would use the records from the 2nd iteration.

...

And this continues till there are no more employees to join on the ManagerID

Then after all the looping, the CTE will return all the records that were found through all those iterations.