Hierarchical Query Pseudocolumns

The hierarchical query pseudocolumns are valid only in hierarchical queries. The hierarchical query pseudocolumns are:

CONNECT_BY_ISCYCLE Pseudocolumn
CONNECT_BY_ISLEAF Pseudocolumn
LEVEL Pseudocolumn

To define a hierarchical relationship in a query, you must use the CONNECT BY clause.

CONNECT_BY_ISCYCLE Pseudocolumn

The CONNECT_BY_ISCYCLE pseudocolumn returns 1 if the current row has a child which is also its ancestor. Otherwise it returns 0.

You can specify CONNECT_BY_ISCYCLE only if you have specified the NOCYCLE parameter of the CONNECT BY clause. NOCYCLE enables Oracle to return the results of a query that would otherwise fail because of a CONNECT BY loop in the data.

CONNECT_BY_ISLEAF Pseudocolumn

The CONNECT_BY_ISLEAF pseudocolumn returns 1 if the current row is a leaf of the tree defined by the CONNECT BY condition. Otherwise it returns 0. This information indicates whether a given row can be further expanded to show more of the hierarchy.

CONNECT_BY_ISLEAF Example The following example shows the first three levels of the hr.employees table, indicating for each row whether it is a leaf row (indicated by 1 in the IsLeaf column) or whether it has child rows (indicated by 0 in the IsLeaf column):

SELECT last_name "Employee", CONNECT_BY_ISLEAF "IsLeaf",
       LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
  FROM employees
  WHERE LEVEL <= 3 AND department_id = 80
  START WITH employee_id = 100
  CONNECT BY PRIOR employee_id = manager_id AND LEVEL <= 4
  ORDER BY "Employee", "IsLeaf";

Employee                      IsLeaf      LEVEL Path
------------------------- ---------- ---------- -------------------------
Abel                               1          3 /King/Zlotkey/Abel
Ande                               1          3 /King/Errazuriz/Ande
Banda                              1          3 /King/Errazuriz/Banda
Bates                              1          3 /King/Cambrault/Bates
Bernstein                          1          3 /King/Russell/Bernstein
Bloom                              1          3 /King/Cambrault/Bloom
Cambrault                          0          2 /King/Cambrault
Cambrault                          1          3 /King/Russell/Cambrault
Doran                              1          3 /King/Partners/Doran
Errazuriz                          0          2 /King/Errazuriz
Fox                                1          3 /King/Cambrault/Fox
. . .

LEVEL Pseudocolumn

For each row returned by a hierarchical query, the LEVEL pseudocolumn returns 1 for a root row, 2 for a child of a root, and so on. A root row is the highest row within an inverted tree. A child row is any nonroot row. A parent row is any row that has children. A leaf row is any row without children. Figure 3-1 shows the nodes of an inverted tree with their LEVEL values.

Figure 3-1 Hierarchical Tree

Description of "Figure 3-1 Hierarchical Tree "