common table expression - SQL Server CTE and recursion example

Question

I never use CTE with recursion. I was just reading an article on it. This article shows employee info with the help of Sql server CTE and recursion. It is basically showing employees and their manager info. I am not able to understand how this query works. Here is the query:

WITH
  cteReports (EmpID, FirstName, LastName, MgrID, EmpLevel)
  AS
  (
    SELECT EmployeeID, FirstName, LastName, ManagerID, 1
    FROM Employees
    WHERE ManagerID IS NULL
    UNION ALL
    SELECT e.EmployeeID, e.FirstName, e.LastName, e.ManagerID,
      r.EmpLevel + 1
    FROM Employees e
      INNER JOIN cteReports r
        ON e.ManagerID = r.EmpID
  )
SELECT
  FirstName + ' ' + LastName AS FullName,
  EmpLevel,
  (SELECT FirstName + ' ' + LastName FROM Employees
    WHERE EmployeeID = cteReports.MgrID) AS Manager
FROM cteReports
ORDER BY EmpLevel, MgrID

Here I am posting about how the output is showing:

I just need to know how it is showing manager first and then his subordinate in a loop. I guess the first sql statement fires only once and that returns all employee ids.

And the second query repeatedly fires, querying the database on which employee exists with the current manager id.

Please explain how the sql statement executes in an internal loop and also tell me the sql execution order. Thanks.

MY 2nd phase of question

;WITH Numbers AS
(
    SELECT n = 1
    UNION ALL
    SELECT n + 1
    FROM Numbers
    WHERE n+1 <= 10
)
SELECT n
FROM Numbers

Q 1) how is the value of N is getting incremented? if the value is assigned to N every time then N value can be incremented but only the first time N value was initialized.

Q 2) CTE and recursion of employee relations:

The moment I add two managers and add a few more employees under the second manager is where the problem starts.

I want to display the first manager detail and in the next rows only those employee details that relate to the subordinate of that manager.

Suppose

ID     Name      MgrID    Level
---    ----      ------   -----
1      Keith      NULL     1
2      Josh       1        2
3      Robin      1        2
4      Raja       2        3
5      Tridip     NULL     1
6      Arijit     5        2
7      Amit       5        2
8      Dev        6        3

I want to display the results in such way with CTE expressions. Please tell me what to modify in my sql which I gave here in order to pull manager-employee relations. Thanks.

I want the output to be like this:

ID          Name   MgrID       nLevel      Family
----------- ------ ----------- ----------- --------------------
1           Keith  NULL        1           1
3           Robin  1           2           1
2           Josh   1           2           1
4           Raja   2           3           1
5           Tridip NULL        1           2
7           Amit   5           2           2
6           Arijit 5           2           2
8           Dev    6           3           2

Is this possible...?

Answer 1

I haven't tested your code, just tried to help you understand how it operates in comment;

WITH
  cteReports (EmpID, FirstName, LastName, MgrID, EmpLevel)
  AS
  (
-->>>>>>>>>>Block 1>>>>>>>>>>>>>>>>>
-- In a rCTE, this block is called an [Anchor]
-- The query finds all root nodes as described by WHERE ManagerID IS NULL
    SELECT EmployeeID, FirstName, LastName, ManagerID, 1
    FROM Employees
    WHERE ManagerID IS NULL
-->>>>>>>>>>Block 1>>>>>>>>>>>>>>>>>
    UNION ALL
-->>>>>>>>>>Block 2>>>>>>>>>>>>>>>>>    
-- This is the recursive expression of the rCTE
-- On the first "execution" it will query data in [Employees],
-- relative to the [Anchor] above.
-- This will produce a resultset, we will call it R{1} and it is JOINed to [Employees]
-- as defined by the hierarchy
-- Subsequent "executions" of this block will reference R{n-1}
    SELECT e.EmployeeID, e.FirstName, e.LastName, e.ManagerID,
      r.EmpLevel + 1
    FROM Employees e
      INNER JOIN cteReports r
        ON e.ManagerID = r.EmpID
-->>>>>>>>>>Block 2>>>>>>>>>>>>>>>>>
  )
SELECT
  FirstName + ' ' + LastName AS FullName,
  EmpLevel,
  (SELECT FirstName + ' ' + LastName FROM Employees
    WHERE EmployeeID = cteReports.MgrID) AS Manager
FROM cteReports
ORDER BY EmpLevel, MgrID

The simplest example of a recursive CTE I can think of to illustrate its operation is;

;WITH Numbers AS
(
    SELECT n = 1
    UNION ALL
    SELECT n + 1
    FROM Numbers
    WHERE n+1 <= 10
)
SELECT n
FROM Numbers

Q 1) how value of N is getting incremented. if value is assign to N every time then N value can be incremented but only first time N value was initialize.

A1: In this case, N is not a variable. N is an alias. It is the equivalent of SELECT 1 AS N. It is a syntax of personal preference. There are 2 main methods of aliasing columns in a CTE in T-SQL. I've included the analog of a simple CTE in Excel to try and illustrate in a more familiar way what is happening.

--  Outside
;WITH CTE (MyColName) AS
(
    SELECT 1
)
-- Inside
;WITH CTE AS
(
    SELECT 1 AS MyColName
    -- Or
    SELECT MyColName = 1  
    -- Etc...
)

Q 2) now here about CTE and recursion of employee relation the moment i add two manager and add few more employee under second manager then problem start. i want to display first manager detail and in the next rows only those employee details will come those who are subordinate of that manager

A2:

Does this code answer your question?

--------------------------------------------
-- Synthesise table with non-recursive CTE
--------------------------------------------
;WITH Employee (ID, Name, MgrID) AS 
(
    SELECT 1,      'Keith',      NULL   UNION ALL
    SELECT 2,      'Josh',       1      UNION ALL
    SELECT 3,      'Robin',      1      UNION ALL
    SELECT 4,      'Raja',       2      UNION ALL
    SELECT 5,      'Tridip',     NULL   UNION ALL
    SELECT 6,      'Arijit',     5      UNION ALL
    SELECT 7,      'Amit',       5      UNION ALL
    SELECT 8,      'Dev',        6   
)
--------------------------------------------
-- Recursive CTE - Chained to the above CTE
--------------------------------------------
,Hierarchy AS
(
    --  Anchor
    SELECT   ID
            ,Name
            ,MgrID
            ,nLevel = 1
            ,Family = ROW_NUMBER() OVER (ORDER BY Name)
    FROM Employee
    WHERE MgrID IS NULL

    UNION ALL
    --  Recursive query
    SELECT   E.ID
            ,E.Name
            ,E.MgrID
            ,H.nLevel+1
            ,Family
    FROM Employee   E
    JOIN Hierarchy  H ON E.MgrID = H.ID
)
SELECT *
FROM Hierarchy
ORDER BY Family, nLevel

Another one sql with tree structure

SELECT ID,space(nLevel+
                    (CASE WHEN nLevel > 1 THEN nLevel ELSE 0 END)
                )+Name
FROM Hierarchy
ORDER BY Family, nLevel