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Hierarchical Queries in Oracle

This articles gives an overview of how to query hierarchical data in Oracle databases using SQL.

Related articles.

Setup

The following table contains hierarchical data.

DROP TABLE tab1 PURGE;

CREATE TABLE tab1 (
  id        NUMBER,
  parent_id NUMBER,
  CONSTRAINT tab1_pk PRIMARY KEY (id),
  CONSTRAINT tab1_tab1_fk FOREIGN KEY (parent_id) REFERENCES tab1(id)
);

CREATE INDEX tab1_parent_id_idx ON tab1(parent_id);

INSERT INTO tab1 VALUES (1, NULL);
INSERT INTO tab1 VALUES (2, 1);
INSERT INTO tab1 VALUES (3, 2);
INSERT INTO tab1 VALUES (4, 2);
INSERT INTO tab1 VALUES (5, 4);
INSERT INTO tab1 VALUES (6, 4);
INSERT INTO tab1 VALUES (7, 1);
INSERT INTO tab1 VALUES (8, 7);
INSERT INTO tab1 VALUES (9, 1);
INSERT INTO tab1 VALUES (10, 9);
INSERT INTO tab1 VALUES (11, 10);
INSERT INTO tab1 VALUES (12, 9);
COMMIT;

Basic Hierarchical Query

In its simplest form a hierarchical query needs a definition of how each child relates to its parent. This is defined using the CONNECT BY .. PRIOR clause, which defines how the current row (child) relates to a prior row (parent). In addition, the START WITH clause can be used to define the root node(s) of the hierarchy. Hierarchical queries come with operators, pseudocolumns and functions to help make sense of the hierarchy.

The following query gives an example of these items based on the previously defined test table.

SET PAGESIZE 20 LINESIZE 110
COLUMN tree FORMAT A20
COLUMN path FORMAT A20

SELECT id,
       parent_id,
       RPAD('.', (level-1)*2, '.') || id AS tree,
       level,
       CONNECT_BY_ROOT id AS root_id,
       LTRIM(SYS_CONNECT_BY_PATH(id, '-'), '-') AS path,
       CONNECT_BY_ISLEAF AS leaf
FROM   tab1
START WITH parent_id IS NULL
CONNECT BY parent_id = PRIOR id
ORDER SIBLINGS BY id;

        ID  PARENT_ID TREE                      LEVEL    ROOT_ID PATH                       LEAF
---------- ---------- -------------------- ---------- ---------- -------------------- ----------
         1            1                             1          1 1                             0
         2          1 ..2                           2          1 1-2                           0
         3          2 ....3                         3          1 1-2-3                         1
         4          2 ....4                         3          1 1-2-4                         0
         5          4 ......5                       4          1 1-2-4-5                       1
         6          4 ......6                       4          1 1-2-4-6                       1
         7          1 ..7                           2          1 1-7                           0
         8          7 ....8                         3          1 1-7-8                         1
         9          1 ..9                           2          1 1-9                           0
        10          9 ....10                        3          1 1-9-10                        0
        11         10 ......11                      4          1 1-9-10-11                     1
        12          9 ....12                        3          1 1-9-12                        1

Cyclic Hierarchical Query

It is possible for a hierarchy to be cyclical, which can represent a problem when querying the data.

-- Create a cyclic reference
UPDATE tab1 SET parent_id = 9 WHERE id = 1;
COMMIT;


SELECT id,
       parent_id,
       RPAD('.', (level-1)*2, '.') || id AS tree,
       level,
       CONNECT_BY_ROOT id AS root_id,
       LTRIM(SYS_CONNECT_BY_PATH(id, '-'), '-') AS path,
       CONNECT_BY_ISLEAF AS leaf
FROM   tab1
START WITH parent_id IS NULL
CONNECT BY parent_id = PRIOR id
ORDER SIBLINGS BY id;
ERROR:
ORA-01436: CONNECT BY loop in user data

To simplify matters, the CONNECT BY NOCYCLE clause tells the database not to traverse cyclical hierarchies. In this case the CONNECT_BY_ISCYCLE function indicates which record is responsible for the cycle.

We can now use the NOCYCLE option and check the results of the CONNECT_BY_ISCYCLE function.

SELECT id,
       parent_id,
       RPAD('.', (level-1)*2, '.') || id AS tree,
       level,
       CONNECT_BY_ROOT id AS root_id,
       LTRIM(SYS_CONNECT_BY_PATH(id, '-'), '-') AS path,
       CONNECT_BY_ISLEAF AS leaf,
       CONNECT_BY_ISCYCLE AS cycle
FROM   tab1
START WITH id = 1
CONNECT BY NOCYCLE parent_id = PRIOR id
ORDER SIBLINGS BY id;

        ID  PARENT_ID TREE                      LEVEL    ROOT_ID PATH                       LEAF      CYCLE
---------- ---------- -------------------- ---------- ---------- -------------------- ---------- ----------
         1          9 1                             1          1 1                             0          0
         2          1 ..2                           2          1 1-2                           0          0
         3          2 ....3                         3          1 1-2-3                         1          0
         4          2 ....4                         3          1 1-2-4                         0          0
         5          4 ......5                       4          1 1-2-4-5                       1          0
         6          4 ......6                       4          1 1-2-4-6                       1          0
         7          1 ..7                           2          1 1-7                           0          0
         8          7 ....8                         3          1 1-7-8                         1          0
         9          1 ..9                           2          1 1-9                           0          1
        10          9 ....10                        3          1 1-9-10                        0          0
        11         10 ......11                      4          1 1-9-10-11                     1          0
        12          9 ....12                        3          1 1-9-12                        1          0

For more information see:

Hope this helps. Regards Tim...

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