Claus on Code

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SQL TIP -> Recursive CTEs

This morning I  had to do some recursive common table experessions. It is acctually relativly simple to do.

A common scenario:

We have a parent child relation in a selfreferencing table. That is common when we have an employee manager relationship or organisation relationship.

Let’s just look at  the  employee manager relationship.

Let’s create an example tmp table like so

CREATE TABLE #Employee (
EmployeeID  INT,
EmployeeName NVARCHAR(50),
ManagerID INT

And let’s insert some test data into the table

INSERT INTO #Employee(EmployeeID, EmployeeName,ManagerID) VALUES (1,'Mr. Manager',NULL)
INSERT INTO #Employee(EmployeeID, EmployeeName,ManagerID) VALUES (2,'Jensen',1)
INSERT INTO #Employee(EmployeeID, EmployeeName,ManagerID) VALUES (3,'Hansen',1)
INSERT INTO #Employee(EmployeeID, EmployeeName,ManagerID) VALUES (4,'Clausen',2)

Where, as you can see,  Mr. Manager is at the top, and Clausen is at the bottom.

We now want to use as recursive common table expression(CTE) to show what level a person is on, in the employee hierarchy.

To do that, we start with the highest level(Where managerID is Null). Then we do a union all with a select which is referencing it self, like so:

WITH EmployeeCTE (EmployeeID,EmployeeName, ManagerID , Level) AS
SELECT EmployeeID, EmployeeName,ManagerID, 0 AS Level
FROM #Employee WHERE
SELECT e.EmployeeID, e.EmployeeName,e.ManagerID, Level+1
FROM #Employee   e
INNER JOIN EmployeeCTE  cte ON cte.EmployeeID=e.ManagerID

And the result can be seen below. This SQL shows what level an employee is on.

EmployeeID  EmployeeName ManagerID   Level
1           Mr. Manager NULL        0
2           Jensen      1           1
3           Hansen      1           1
4           Clausen     2           2

If we want to see an employees way to the top, we can select from the same CTE like so:

SELECT  lvl0.EmployeeName as lvl0Name,  lvl1.EmployeeName AS lvl1Name,      lvl2.EmployeeName AS lvl2Name FROM EmployeeCTE lvl2
INNER JOIN EmployeeCTE lvl1 ON lvl1.EmployeeID=lvl2.ManagerID
INNER JOIN EmployeeCTE lvl0 ON lvl0.EmployeeID=lvl1.ManagerID
WHERE lvl2.Level=2

And the result:

lvl0Name     lvl1Name     lvl2Name
Mr. Manager Jensen        Clausen

Now you know how to do recursive CTE 🙂

Have fun

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