Organization tree in the sample application v17
To see an example of an organizational tree, consider the emp
table of the sample application. The rows of the emp
table form a hierarchy based on the mgr
column, which contains the employee number of the employee’s manager. Each employee has at most one manager. KING
is the president of the company. He has no manager, therefore KING’s mgr
column is null. Also, it's possible for an employee to act as a manager for more than one employee. This relationship forms a typical tree-structured hierarchical organization chart:
To form a hierarchical query based on this relationship, the SELECT
command includes the clause CONNECT BY PRIOR empno = mgr
. For example, given the company president, KING
, with employee number 7839
, any employee whose mgr
column is 7839
reports directly to KING
. This relationship is true for JONES
, BLAKE
, and CLARK
, the child nodes of KING
. Similarly, for employee JONES
, any other employee with mgr
column equal to 7566
is a child node of JONES
. These nodes are SCOTT
and FORD
in the example.
The top of the organization chart is KING
, so there is one root node in this tree. The START WITH mgr IS NULL
clause selects only KING
as the initial root node.
The complete SELECT
command is:
SELECT ename, empno, mgr FROM emp START WITH mgr IS NULL CONNECT BY PRIOR empno = mgr;
The rows in the query output traverse each branch from the root to leaf moving in a top-to-bottom, left-to-right order:
ename | empno | mgr -------+-------+------ KING | 7839 | JONES | 7566 | 7839 SCOTT | 7788 | 7566 ADAMS | 7876 | 7788 FORD | 7902 | 7566 SMITH | 7369 | 7902 BLAKE | 7698 | 7839 ALLEN | 7499 | 7698 WARD | 7521 | 7698 MARTIN | 7654 | 7698 TURNER | 7844 | 7698 JAMES | 7900 | 7698 CLARK | 7782 | 7839 MILLER | 7934 | 7782 (14 rows)