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Recursive Subquery Factoring : Hierarchical Queries Using Recursive WITH Clauses

This articles gives an overview of how to query hierarchical data in Oracle databases using recursive subquery factoring. This functionality was introduced in Oracle 11g Release 2, giving an alternative to the method of performing hierarchical queries from previous versions.

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

A recursive subquery factoring clause must contain two query blocks combined by a UNION ALL set operator. The first block is known as the anchor member, which can not reference the query name. It can be made up of one or more query blocks combined by the UNION ALL, UNION, INTERSECT or MINUS set operators. The second query block is known as the recursive member, which must reference the query name once.

The following query uses a recursive WITH clause to perform a tree walk. The anchor member queries the root nodes by testing for records with no parents. The recursive member successively adds the children to the root nodes.

SET PAGESIZE 20 LINESIZE 110

WITH t1(id, parent_id) AS (
  -- Anchor member.
  SELECT id,
         parent_id
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SELECT id,
       parent_id
FROM   t1;

        ID  PARENT_ID
---------- ----------
         1
         2          1
         7          1
         9          1
         3          2
         4          2
         8          7
        10          9
        12          9
         5          4
         6          4
        11         10

The ordering of the rows is specified using the SEARCH clause, which can use two methods.

The following queries show the result of these differing ordering methods.

WITH t1(id, parent_id) AS (
  -- Anchor member.
  SELECT id,
         parent_id
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH BREADTH FIRST BY id SET order1
SELECT id,
       parent_id
FROM   t1
ORDER BY order1;

        ID  PARENT_ID
---------- ----------
         1
         2          1
         7          1
         9          1
         3          2
         4          2
         8          7
        10          9
        12          9
         5          4
         6          4
        11         10


WITH t1(id, parent_id) AS (
  -- Anchor member.
  SELECT id,
         parent_id
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
SELECT id,
       parent_id
FROM   t1
ORDER BY order1;

        ID  PARENT_ID
---------- ----------
         1
         2          1
         3          2
         4          2
         5          4
         6          4
         7          1
         8          7
         9          1
        10          9
        11         10
        12          9

Implement Equivalent of LEVEL

The following example shows how we can determine the LEVEL of the hierarchy the current row resides in.

COLUMN tree FORMAT A20

WITH t1(id, parent_id, lvl) AS (
  -- Anchor member.
  SELECT id,
         parent_id,
         1 AS lvl
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id,
         lvl+1
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
SELECT id,
       parent_id,
       RPAD('.', (lvl-1)*2, '.') || id AS tree,
       lvl
FROM t1
ORDER BY order1;

        ID  PARENT_ID TREE                        LVL
---------- ---------- -------------------- ----------
         1            1                             1
         2          1 ..2                           2
         3          2 ....3                         3
         4          2 ....4                         3
         5          4 ......5                       4
         6          4 ......6                       4
         7          1 ..7                           2
         8          7 ....8                         3
         9          1 ..9                           2
        10          9 ....10                        3
        11         10 ......11                      4
        12          9 ....12                        3

Implement Equivalent of CONNECT_BY_ROOT

The next example shows how to mimic CONNECT_BY_ROOT functionality.

WITH t1(id, parent_id, lvl, root_id) AS (
  -- Anchor member.
  SELECT id,
         parent_id,
         1 AS lvl,
         id AS root_id
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id,
         lvl+1,
         t1.root_id
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
SELECT id,
       parent_id,
       RPAD('.', (lvl-1)*2, '.') || id AS tree,
       lvl,
       root_id
FROM t1
ORDER BY order1;

        ID  PARENT_ID TREE                        LVL    ROOT_ID
---------- ---------- -------------------- ---------- ----------
         1            1                             1          1
         2          1 ..2                           2          1
         3          2 ....3                         3          1
         4          2 ....4                         3          1
         5          4 ......5                       4          1
         6          4 ......6                       4          1
         7          1 ..7                           2          1
         8          7 ....8                         3          1
         9          1 ..9                           2          1
        10          9 ....10                        3          1
        11         10 ......11                      4          1
        12          9 ....12                        3          1

Implement Equivalent of SYS_CONNECT_BY_PATH

The following example shows how to mimic the SYS_CONNECT_BY_PATH functionality.

COLUMN path FORMAT A20

WITH t1(id, parent_id, lvl, root_id, path) AS (
  -- Anchor member.
  SELECT id,
         parent_id,
         1 AS lvl,
         id AS root_id,
         TO_CHAR(id) AS path
  FROM   tab1
  WHERE  parent_id IS NULL
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id,
         lvl+1,
         t1.root_id,
         t1.path || '-' || t2.id AS path
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
SELECT id,
       parent_id,
       RPAD('.', (lvl-1)*2, '.') || id AS tree,
       lvl,
       root_id,
       path
FROM t1
ORDER BY order1;

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

Cyclic Hierarchical Query (NOCYCLE and CONNECT_BY_ISCYCLE)

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;


WITH t1(id, parent_id, lvl, root_id, path) AS (
  -- Anchor member.
  SELECT id,
         parent_id,
         1 AS lvl,
         id AS root_id,
         TO_CHAR(id) AS path
  FROM   tab1
  WHERE  id = 1
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id,
         lvl+1,
         t1.root_id,
         t1.path || '-' || t2.id AS path
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
SELECT id,
       parent_id,
       RPAD('.', (lvl-1)*2, '.') || id AS tree,
       lvl,
       root_id,
       path
FROM t1
ORDER BY order1;
     *
ERROR at line 27:
ORA-32044: cycle detected while executing recursive WITH query

The NOCYCLE and CONNECT_BY_ISCYCLE functionality is replicated using the CYCLE clause. By specifying this clause, the cycle is detected and the recursion stops, with the cycle column set to the specified value, to indicate the row where the cycle is detected. Unlike the CONNECT BY NOCYCLE method, which stops at the row before the cycle, this method stops at the row after the cycle.

WITH t1(id, parent_id, lvl, root_id, path) AS (
  -- Anchor member.
  SELECT id,
         parent_id,
         1 AS lvl,
         id AS root_id,
         TO_CHAR(id) AS path
  FROM   tab1
  WHERE  id = 1
  UNION ALL
  -- Recursive member.
  SELECT t2.id,
         t2.parent_id,
         lvl+1,
         t1.root_id,
         t1.path || '-' || t2.id AS path
  FROM   tab1 t2, t1
  WHERE  t2.parent_id = t1.id
)
SEARCH DEPTH FIRST BY id SET order1
CYCLE id SET cycle TO 1 DEFAULT 0
SELECT id,
       parent_id,
       RPAD('.', (lvl-1)*2, '.') || id AS tree,
       lvl,
       root_id,
       path,
       cycle
FROM t1
ORDER BY order1;

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

For more information see:

Hope this helps. Regards Tim...

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