How to retrieve hierarchical data from SQL table

Question

I have 2 tables : T_Employees and T_Projects

Every project has different number of employees assigned. What i need to do, is to get hierarchical structure of each employ

Answer 1

You can travers a hierarchy with a recursive CTE

This is the classical top down CTE:

WITH EmployeesHierarchy AS
(
    SELECT ID,[Level],Employee,Department,Code,MasterId
    FROM T_Employees
    WHERE [Level]=1
    UNION ALL
    SELECT nextLevel.ID,nextLevel.[Level],nextLevel.Employee,nextLevel.Department,nextLevel.Code,nextLevel.MasterId
    FROM EmployeesHierarchy AS recCall
    INNER JOIN T_Employees AS nextLevel ON nextLevel.[Level]=recCall.[Level]+1 AND nextLevel.MasterId=recCall.ID
)
SELECT * FROM EmployeesHierarchy
ORDER BY [Level],MasterId
GO

And now the other way round: I start with the employees mentioned in the project and move up the list until there is no parentId any more. The Project's data gotten in the first part of the CTE are just passed through to show up in all rows.

WITH EmployeesHierarchy AS
(
    SELECT p.ID AS p_ID,p.ProjectId,e.ID AS e_ID,[Level],e.Employee,e.Department,e.Code,e.MasterId
    FROM T_Projects AS p 
        INNER JOIN T_Employees AS e ON p.EmployeeId=e.ID
    UNION ALL
    SELECT recCall.p_ID,recCall.ProjectId,nextLevel.ID,nextLevel.[Level],nextLevel.Employee,nextLevel.Department,nextLevel.Code,nextLevel.MasterId
    FROM EmployeesHierarchy AS recCall
    INNER JOIN T_Employees AS nextLevel ON nextLevel.ID=recCall.MasterId
)
SELECT * FROM EmployeesHierarchy
--WHERE ProjectId=456
ORDER BY [Level]

The result

+------+-----------+------+-------+------------+------------+------+----------+
| p_ID | ProjectId | e_ID | Level | Employee   | Department | Code | MasterId |
+------+-----------+------+-------+------------+------------+------+----------+
| 3    | 23        | 1    | 1     | Thomas S.  | A          | 1-4  | NULL     |
+------+-----------+------+-------+------------+------------+------+----------+
| 2    | 456       | 3    | 1     | Simone S.  | A          | 1-3  | NULL     |
+------+-----------+------+-------+------------+------------+------+----------+
| 1    | 456       | 1    | 1     | Thomas S.  | A          | 1-4  | NULL     |
+------+-----------+------+-------+------------+------------+------+----------+
| 2    | 456       | 6    | 2     | Loris P.   | B          | 2-15 | 3        |
+------+-----------+------+-------+------------+------------+------+----------+
| 1    | 456       | 4    | 2     | Stefan K.  | B          | 2-18 | 1        |
+------+-----------+------+-------+------------+------------+------+----------+
| 3    | 23        | 4    | 2     | Stefan K.  | B          | 2-18 | 1        |
+------+-----------+------+-------+------------+------------+------+----------+
| 3    | 23        | 7    | 3     | Lennon I.  | B          | 2-19 | 4        |
+------+-----------+------+-------+------------+------------+------+----------+
| 1    | 456       | 8    | 3     | Kerim K.   | C          | 2-66 | 4        |
+------+-----------+------+-------+------------+------------+------+----------+
| 2    | 456       | 9    | 3     | Ilmas Y.   | C          | 2-59 | 6        |
+------+-----------+------+-------+------------+------------+------+----------+
| 1    | 456       | 10   | 4     | Innes Y.   | D          | 3-89 | 8        |
+------+-----------+------+-------+------------+------------+------+----------+
| 2    | 456       | 12   | 4     | Fatih O.   | I          | 3-32 | 9        |
+------+-----------+------+-------+------------+------------+------+----------+
| 3    | 23        | 11   | 4     | Andreas U. | E          | 3-63 | 7        |
+------+-----------+------+-------+------------+------------+------+----------+