Way to check for recursive foreign keys

Question

Say I have a table named EMPLOYEE, and the table has a field called ManagerID. This field is a FK to another record of the EMPLOYEE table of who their manager is. What is an ideal way to verify that there is no circular reference (recursive foreign keys)?

EDIT - Let me add that every employee does NOT need a manager. There will be some employee records that do not have their ManagerID field set.

*Example: User 123 has User 345 as their manager. User 345 has User 567 as their manager. For some crazy reason, User 567 has User 123 as their manager.

I want to check if User 345 is in a circular reference.*

What is the best way to figure this out? A simple query? Business logic code (C#/VB.NET)?

What have other people done?

Example table schema -

-- Employee Table --
UserID (int - PK)
ManagerID (int - FK to another UserID; can be null)

Answer 1

Here's a few starters - they either use recursive CTEs or cursors:

http://blogs.msdn.com/b/sqlazure/archive/2010/07/01/10033575.aspx

http://social.msdn.microsoft.com/Forums/en-ZA/transactsql/thread/50a0bd26-87c8-4197-84f9-5fb1dfd792b6

Answer 2

If you are dealing with SQL server, here is the solution that matches your requirement; You need to write CTE to find cyclic references;

Below query will give you user IDs having cyclic reference

WITH TRACKER AS
(


SELECT U.id,U.manager_id,CAST(','+cast(U.id as varchar(36))+',' as varchar(max)) as Path,0 as Cycle FROM User_table AS U
    UNION ALL
    SELECT U.id,U.manager_id, TRACKER.Path + CAST(U.id AS varchar(36))+',', case WHEN TRACKER.Path LIKE '%,'+CAST(U.id AS VARCHAR(36)) + ',%' then 1 else 0 end   FROM User_table AS U

 INNER JOIN TRACKER ON U.manager_id = TRACKER.id WHERE TRACKER.Cycle=0 
)

SELECT TRACKER.id from TRACKER where Cycle= 1;