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Need help on understanding recursive CTE

Posted on 2013-01-17
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Last Modified: 2013-01-17
Hello,
I am studying recursive CTE and I am having trouble understanding the recursive portion of the cte.  Below is the example.  I want to make sure that I understand what's going on.  I understand the anchor member portion but am not sure how the recursive portion works.  I've read the explanation but am still having trouble with the concept.  I'm studying on my own.

USE Examples;

WITH EmployeesCTE AS
(
		-- Anchor member
		SELECT EmployeeID, 
			FirstName + ' ' + LastName As EmployeeName, 
			1 As Rank
		FROM Employees
		WHERE ManagerID IS NULL
	UNION ALL
		-- Recursive member
		SELECT Employees.EmployeeID, 
			FirstName + ' ' + LastName, 
			Rank + 1
		FROM Employees
			JOIN EmployeesCTE
			ON Employees.ManagerID = EmployeesCTE.EmployeeID
)
SELECT *
FROM EmployeesCTE
ORDER BY Rank, EmployeeID

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Question by:chtullu135
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Author Comment

by:chtullu135
ID: 38788649
I think what is happening is that "Join EmployeesCTE" is referring to the previous pass .
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LVL 45

Expert Comment

by:Kdo
ID: 38788695
Hit chtullu135,

There are a couple of articles on the DB2 forum that explain recursive SQL very well.  The only difference between the DB2 and SQL Server variants is how string parameters are sized (it's explained in the articles).  The only other significant difference is that DB2 uses the double pipe (||) for string concatenation and SQL Server uses the plus sign (+).

  http://www.experts-exchange.com/Database/DB2/A_3629-Recursive-SQL-in-DB2-Converting-Columns-to-Rows.html

  http://www.experts-exchange.com/Database/DB2/A_3618-Recursive-SQL-in-DB2-Converting-rows-to-columns.html


Briefly, the upper sub-query in the CTE establishes the starting point of the recursive query.  The lower sub-query in the CTE processes all of the rows (except the first row) and defines when the recursion is complete.


In your query, the upper sub-query selects all employees without a manager.  The lower sub-query doesn't seem to select rows in a meaningful fashion.  What is it trying to do?


Kent
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Author Comment

by:chtullu135
ID: 38788822
Hello Kent,
The lower subquery returns each employee according the their level in the organization.  Below is the list of Employee

Empid                                             Managerid
1                Smith      Cindy               NULL
2                Jones      Elmer                 1
3                Simonian      Ralph         2
4               Hernandez      Olivia         9
5               Aaronsen      Robert         4
6               Watson      Denise                 8
7                Hardy      Thomas                2
8      O'Leary      Rhea      9
9      Locario      Paulo      1
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Author Comment

by:chtullu135
ID: 38788876
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LVL 45

Accepted Solution

by:
Kdo earned 500 total points
ID: 38788905
Ok.

Using your sample data, the upper sub-query selects the row for Cindy Smith -- the only row with NULL for the manager id.

The lower sub-query takes that row and joins the employees table to it and filters in a way that the employees that are Cindy's direct reports are selected.  (Paulo Locario and Elmer Jones.)

The next iteration selects the rows for people that report to Locario and Jones.

Note that the selections are not linear.  People want to think of recursive SQL as iterating through a list.  In reality, SQL is taking some number of rows of data and generating more rows of data.  (All level 1 rows are generated at the same time, all level 2 rows are generated at the same time, etc.)

The other real gotcha is that ALL of the rows that are generated are part of the final result set.  You may need to filter or sort them differently for your own needs.  In this case, I believe that you'd be well served for the DTE to pass the employeeID and managerID values.


Kent
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Author Comment

by:chtullu135
ID: 38788927
Thanks Kent.  My problem was that I was thinking the the recursion was iterating through a list instead of a set of rows.
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Author Closing Comment

by:chtullu135
ID: 38791822
Thanks again for your help
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