Subquery in FROM clause - sql

Question

Looking around in the (now-discontinued) documentation and found this example:
Subquery in FROM clause
A subquery in a FROM clause acts similarly to a temporary table that is generated during the execution of a query and lost afterwards.
SELECT Managers.Id, Employees.Salary
FROM (
SELECT Id
FROM Employees
WHERE ManagerId IS NULL
) AS Managers
JOIN Employees ON Managers.Id = Employees.Id
(Excerpted from Subqueries - Subquery in FROM clause. The original author was Phrancis. Attribution details can be found on the contributor page. The source is licenced under CC BY-SA 3.0 and may be found in the Documentation archive. Reference topic ID: 1030 and example ID: 3327.)
My question is:
why using an extra ManagerId. An Id column is already in
Employees table,
why have the extra ManagerId to be null for a manager (ok it wants to be a joke).
My opinion:
despite the upvotes, something is wrong with this is example,
Tables with example data would be nice to see on the fly how it's working. One table with start data, one table temporary SELECT and one table the
resultset.
Edit: Thanks to all contributors for their answers!
#Alex K.: That is my point of view "it is not something one would actually use". But people, who wants to learn SQL, might think, that it is good practice, because it is in the documentation here.,
#Nebi: Thanks for the point that one would write it simpler to get the same result.
#Unnikrishnan R: "showcase how the sub query works" does in my eyes not only mean that it is fully functional but additional that it makes sense. If I get things simpler, why doing it the errorprone hard way.
#me: should have titled it "let's discuss sql documentation" or like that ;)

Answer 1

Let us consider a situation where Employee table holds all employees including their managers in which employee has an Id, and there is also a column for the manager Id (which can be null). This can be the point of view ,who was writing that SQL queries.
For Example,
+----+-------+--------+-----------+
| Id | Name | Salary | ManagerId |
+----+-------+--------+-----------+
| 1 | Joe | 70000 | 3 |
| 2 | Henry | 80000 | 4 |
| 3 | Sam | 60000 | NULL |
| 4 | Max | 90000 | NULL |
+----+-------+--------+-----------+
why have the extra ManagerId to be null for a manager --
getting the employees that are not managers

Answer 2

It is just an example how to do/ use Subqueries.
To your questions:
why using an extra ManagerId. An Id column is already in Employees
table
First of all ManagerId and Id are different columns of the table Employees. So there is a difference between them. But you might reffering to the Id of the Subquery Managers and the Id of the joined table Employees.
Then you need to define which Id you are using. Else you get the Error for ambigiuos column. In this example you to specify either the Subqueries Id which is Managers.Id or the Id of the joined table Employees (Employees.Id). Which one you choose is totally regardless because you use INNER JOIN one the Id.
why have the extra ManagerId to be null for a manager (ok it wants
to be a joke).
This is because of getting all the Employees that have are not managers. You are right about saying this could be done easier or in other form. For instance:
SELECT Id, Salary
FROM Employees
WHERE ManagerId IS NULL
This probably gets the same result as in the original. But the example is not about that, it is about the structure of a subquery.

Answer 3

why using an extra ManagerId. An Id column is already in Employees table
Consider you are having an employee table and you also wanted to keep the manager information in the same table.so apart from the ID column you need to add another column to keep the managerid.
why have the extra ManagerId to be null for a manager (ok it wants to be a joke).
The query is just to showcase how the sub query works. In this case subquery retrieves the manager from the Employee table (managerID is null) then join those id's with Employee table in the outer query to get the salary of each managers.