A recursive CTE will reference itself. As we should know in recursion we should have a "exit" clause. This is achieved in a recursive CTE with an anchor member. The anchor member doesn't reference the CTE - whereas the recursive member of the CTE will do.
Our CTE is thus made up of two elements - the anchor member and the recursive member. These two parts are combined using a SET operator - normally UNION or UNION ALL.
WITH cte_name ( column_names ... ) AS ( CTE_query_for_anchor_member UNION [ALL] CTE_query_for_recursive_member ) SELECT * FROM cte_name -- Or other statement to use CTEI will use the Northwind database Employees table.
SELECT EmployeeID, LastName, Title, ReportsTo FROM EmployeesGiving us the output
EmployeeID LastName Title ReportsTo ----------- -------------------- ------------------------------ ----------- 1 Davolio Sales Representative 2 2 Fuller Vice President, Sales NULL 3 Leverling Sales Representative 2 4 Peacock Sales Representative 2 5 Buchanan Sales Manager 2 6 Suyama Sales Representative 5 7 King Sales Representative 5 8 Callahan Inside Sales Coordinator 2 9 Dodsworth Sales Representative 5That's the raw data - let's try and put this into a "hierarchy" of the employees and there managers. Each employee's manager is defined by the ReportsTo column, so we can join on this.
SELECT E.EmployeeID, E.LastName, E.Title, E.ReportsTo, M.LastName--, M.Title FROM Employees M JOIN Employees E ON (E.ReportsTo = M.EmployeeID)Now showing the manager's name (through the JOIN)
EmployeeID LastName Title ReportsTo LastName ----------- -------------------- ------------------------------ ----------- -------------------- 1 Davolio Sales Representative 2 Fuller 3 Leverling Sales Representative 2 Fuller 4 Peacock Sales Representative 2 Fuller 5 Buchanan Sales Manager 2 Fuller 6 Suyama Sales Representative 5 Buchanan 7 King Sales Representative 5 Buchanan 8 Callahan Inside Sales Coordinator 2 Fuller 9 Dodsworth Sales Representative 5 BuchananWe need to tweak this a little as the top manager (Fuller - Vice President of Sales) is missing, we need an Outer Join.
SELECT E.EmployeeID, E.LastName, E.Title, E.ReportsTo, M.LastName FROM Employees M RIGHT OUTER JOIN Employees E ON (E.ReportsTo = M.EmployeeID)Now showing Fuller - with Nulls for who he reports to.
EmployeeID LastName Title ReportsTo LastName ----------- -------------------- ------------------------------ ----------- -------------------- 1 Davolio Sales Representative 2 Fuller 2 Fuller Vice President, Sales NULL NULL 3 Leverling Sales Representative 2 Fuller 4 Peacock Sales Representative 2 Fuller 5 Buchanan Sales Manager 2 Fuller 6 Suyama Sales Representative 5 Buchanan 7 King Sales Representative 5 Buchanan 8 Callahan Inside Sales Coordinator 2 Fuller 9 Dodsworth Sales Representative 5 BuchananThere we go. Let's do this with a recursive CTE. The anchor condition will be when the ReportsTo column is NULL - i.e. when we have an employee who doesn't report to anyone else.
SELECT ReportsTo, EmployeeID, LastName, FirstName, Title FROM Employees WHERE ReportsTo IS NULLDoesn't seem like much - but let's think of this as a separate query. Imagine our CTE is called CTEEmployees and this SQL refers to it
SELECT e.ReportsTo, e.EmployeeID, e.LastName, e.FirstName, e.Title
FROM Employees AS e
INNER JOIN CTEEmployees AS d
ON e.ReportsTo = d.EmployeeID
Putting this together into a CTE and a query to use the CTE we would haveWITH CTEEmployees(ReportsTo, EmployeeID, LastName, FirstName, Title) AS
(
SELECT ReportsTo, EmployeeID, LastName, FirstName, Title
FROM Employees
WHERE ReportsTo IS NULL
UNION ALL
SELECT e.ReportsTo, e.EmployeeID, e.LastName, e.FirstName, e.Title
FROM Employees AS e
INNER JOIN CTEEmployees AS d
ON e.ReportsTo = d.EmployeeID
)
SELECT EmployeeID, LastName, FirstName, Title, ReportsTo
FROM CTEEmployees
ORDER BY EmployeeID
Giving the resultsEmployeeID LastName FirstName Title ReportsTo ----------- -------------------- ---------- ------------------------------ ----------- 1 Davolio Nancy Sales Representative 2 2 Fuller Andrew Vice President, Sales NULL 3 Leverling Janet Sales Representative 2 4 Peacock Margaret Sales Representative 2 5 Buchanan Steven Sales Manager 2 6 Suyama Michael Sales Representative 5 7 King Robert Sales Representative 5 8 Callahan Laura Inside Sales Coordinator 2 9 Dodsworth Anne Sales Representative 5I'm not sure it was worth the hassle to get there when an outer join did this for me. But let's add another column to our output
WITH CTEEmployees(ReportsTo, EmployeeID, LastName, FirstName, Title, EmployeeLevel) AS
(
SELECT ReportsTo, EmployeeID, LastName, FirstName, Title, 0
FROM Employees
WHERE ReportsTo IS NULL
UNION ALL
SELECT e.ReportsTo, e.EmployeeID, e.LastName, e.FirstName, e.Title, d.EmployeeLevel + 1
FROM Employees AS e
INNER JOIN CTEEmployees AS d
ON e.ReportsTo = d.EmployeeID
)
SELECT EmployeeID, LastName, FirstName, Title, ReportsTo, EmployeeLevel
FROM CTEEmployees
ORDER BY EmployeeID
Giving us the same output with a column showing the levelEmployeeID LastName FirstName Title ReportsTo EmployeeLevel ----------- -------------------- ---------- ------------------------------ ----------- ------------- 1 Davolio Nancy Sales Representative 2 1 2 Fuller Andrew Vice President, Sales NULL 0 3 Leverling Janet Sales Representative 2 1 4 Peacock Margaret Sales Representative 2 1 5 Buchanan Steven Sales Manager 2 1 6 Suyama Michael Sales Representative 5 2 7 King Robert Sales Representative 5 2 8 Callahan Laura Inside Sales Coordinator 2 1 9 Dodsworth Anne Sales Representative 5 2This extra column would be useful when displaying the output.
The actual use of recursive of CTE is going to be limited to data that is stored in a hierarchical structure - i.e. with self joins.
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