This is follow up blog post of my earlier blog post on the same subject - SQL SERVER – Introduction to Hierarchical Query using a Recursive CTE – A Primer. In the article we discussed various basics terminology of the CTE. The article further covers following important concepts of common table expression.

• What is a Common Table Expression (CTE)
• Building a Recursive CTE
• Identify the Anchor and Recursive Query
• Add the Anchor and Recursive query to a CTE
• Add an expression to track hierarchical level
• Add a self-referencing INNER JOIN statement

Above six are the most important concepts related to CTE and SQL Server.  There are many more things one has to learn but without beginners fundamentals one can’t learn the advanced  concepts. Let us have small quiz and check how many of you get the fundamentals right.

Quiz

1) You have an employee table with the following data.

EmpID	FirstName	LastName	MgrID
1	David	Kennson	11
2	Eric	Bender	11
3	Lisa	Kendall	4
4	David	Lonning	11
5	John	Marshbank	4
6	James	Newton	3
7	Sally	Smith	NULL

You need to write a recursive CTE that shows the EmpID, FirstName, LastName, MgrID, and employee level. The CEO should be listed at Level 1. All people who work for the CEO will be listed at Level 2. All of the people who work for those people will be listed at Level 3. Which CTE code will achieve this result?

1. WITH EmpList AS
   (SELECT Boss.EmpID, Boss.FName, Boss.LName, Boss.MgrID,
   1 AS Lvl
   FROM Employee AS Boss WHERE Boss.MgrID IS NULL
   UNION ALL
   SELECT E.EmpID, E.FirstName, E.LastName, E.MgrID, EmpList.Lvl + 1
   FROM Employee AS E INNER JOIN EmpList
   ON E.MgrID = EmpList.EmpID)
   SELECT * FROM EmpList
2. WITH EmpListAS
   (SELECT EmpID, FirstName, LastName, MgrID, 1 as Lvl
   FROM Employee WHERE MgrID IS NULL
   UNION ALL
   SELECT EmpID, FirstName, LastName, MgrID, 2 as Lvl )
   SELECT * FROM BossList
3. WITH EmpList AS
   (SELECT EmpID, FirstName, LastName, MgrID, 1 as Lvl
   FROM Employee WHERE MgrID is NOT NULL
   UNION
   SELECT EmpID, FirstName, LastName, MgrID, BossList.Lvl + 1
   FROM Employee INNER JOIN EmpList BossList
   ON Employee.MgrID = BossList.EmpID)
   SELECT * FROM EmpList

2) You have a table named Employee. The EmployeeID of each employee’s manager is in the ManagerID column. You need to write a recursive query that produces a list of employees and their manager. The query must also include the employee’s level in the hierarchy. You write the following code segment:

WITH EmployeeList (EmployeeID, FullName, ManagerName, Level)
AS (
–PICK ANSWER CODE HERE
)

1. SELECT EmployeeID, FullName, ” AS [ManagerID], 1 AS [Level]
   FROM Employee
   WHERE ManagerID IS NULL
   UNION ALL
   SELECT emp.EmployeeID, emp.FullName mgr.FullName, 1 + 1 AS [Level]
   FROM Employee emp JOIN Employee mgr
   ON emp.ManagerID = mgr.EmployeeId
2. SELECT EmployeeID, FullName, ” AS [ManagerID], 1 AS [Level]
   FROM Employee
   WHERE ManagerID IS NULL
   UNION ALL
   SELECT emp.EmployeeID, emp.FullName, mgr.FullName, mgr.Level + 1
   FROM EmployeeList mgr JOIN Employee emp
   ON emp.ManagerID = mgr.EmployeeId

Reference: Pinal Dave (http://blog.sqlauthority.com)

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