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Oracle9i SQL Reference
Release 2 (9.2)

Part Number A96540-02
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Hierarchical Queries

If a table contains hierarchical data, then you can select rows in a hierarchical order using the hierarchical query clause:


Text description of queries3.gif follows
Text description of hierarchical_query_clause

The manner in which Oracle processes a WHERE clause (if any) in a hierarchical query depends on whether the WHERE clause contains a join:

Oracle uses the information from the hierarchical query clause to form the hierarchy using the following steps:

  1. Oracle processes the WHERE clause either before or after the CONNECT BY clause depending on whether the WHERE clause contains any join predicates (as described in the preceding bullet list).
  2. Oracle selects the root row(s) of the hierarchy--those rows that satisfy the START WITH condition.
  3. Oracle selects the child rows of each root row. Each child row must satisfy the condition of the CONNECT BY condition with respect to one of the root rows.
  4. Oracle selects successive generations of child rows. Oracle first selects the children of the rows returned in step 3, and then the children of those children, and so on. Oracle always selects children by evaluating the CONNECT BY condition with respect to a current parent row.
  5. If the query contains a WHERE clause without a join, then Oracle eliminates all rows from the hierarchy that do not satisfy the condition of the WHERE clause. Oracle evaluates this condition for each row individually, rather than removing all the children of a row that does not satisfy the condition.
  6. Oracle returns the rows in the order shown in Figure 8-1. In the diagram, children appear below their parents. For an explanation of hierarchical trees, see Figure 2-1, "Hierarchical Tree".

Figure 8-1 Hierarchical Queries

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Text description of the illustration queries4.gif

To find the children of a parent row, Oracle evaluates the PRIOR expression of the CONNECT BY condition for the parent row and the other expression for each row in the table. Rows for which the condition is true are the children of the parent. The CONNECT BY condition can contain other conditions to further filter the rows selected by the query. The CONNECT BY condition cannot contain a subquery.

If the CONNECT BY condition results in a loop in the hierarchy, then Oracle returns an error. A loop occurs if one row is both the parent (or grandparent or direct ancestor) and a child (or a grandchild or a direct descendent) of another row.


In a hierarchical query, do not specify either ORDER BY or GROUP BY, as they will destroy the hierarchical order of the CONNECT BY results. If you want to order rows of siblings of the same parent, then use the ORDER SIBLINGS BY clause. See order_by_clause.


The following hierarchical query uses the CONNECT BY clause to define the relationship between employees and managers:

SELECT employee_id, last_name, manager_id
   FROM employees
   CONNECT BY PRIOR employee_id = manager_id;

----------- ------------------------- ----------
        101 Kochhar                          100
        108 Greenberg                        101
        109 Faviet                           108
        110 Chen                             108
        111 Sciarra                          108
        112 Urman                            108
        113 Popp                             108
        200 Whalen                           101

The next example is similar to the preceding example, but uses the LEVEL pseudocolumn to show parent and child rows:

SELECT employee_id, last_name, manager_id, LEVEL
   FROM employees
   CONNECT BY PRIOR employee_id = manager_id;

----------- ------------------------- ---------- ----------
        101 Kochhar                          100          1
        108 Greenberg                        101          2
        109 Faviet                           108          3
        110 Chen                             108          3
        111 Sciarra                          108          3
        112 Urman                            108          3
        113 Popp                             108          3

Finally, the next example adds a START WITH clause to specify a root row for the hierarchy, and an ORDER BY clause using the SIBLINGS keyword to preserve ordering within the hierarchy:

SELECT last_name, employee_id, manager_id, LEVEL
      FROM employees
      START WITH employee_id = 100
      CONNECT BY PRIOR employee_id = manager_id
      ORDER SIBLINGS BY last_name;

------------------------- ----------- ---------- ----------
King                              100                     1
Cambrault                         148        100          2
Bates                             172        148          3
Bloom                             169        148          3
Fox                               170        148          3
Kumar                             173        148          3
Ozer                              168        148          3
Smith                             171        148          3
De Haan                           102        100          2
Hunold                            103        102          3
Austin                            105        103          4
Ernst                             104        103          4
Lorentz                           107        103          4
Pataballa                         106        103          4
Errazuriz                         147        100          2
Ande                              166        147          3
Banda                             167        147          3
See Also:
  • LEVEL for a discussion of how the LEVEL pseudocolumn operates in a hierarchical query
  • SYS_CONNECT_BY_PATH for information on retrieving the path of column values from root to node
  • order_by_clause for more information on the SIBLINGS keyword of ORDER BY clauses