CONNECT BY LOOP
A loop in user data generates an �ORA-01436: CONNECT BY loop in user data� error. A loop is defined in the Oracle Database SQL Language Reference (11.1) documentation as:
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
WITH
T
AS
(
SELECT
'JOHN' EMPLOYEE,
'JACK' MANAGER
FROM
DUAL
UNION ALL
SELECT
'JACK' EMPLOYEE,
'JOHN' MANAGER
FROM
DUAL
)
SELECT
EMPLOYEE,
MANAGER
FROM
T
CONNECT BY
PRIOR EMPLOYEE = MANAGER;
ERROR:
ORA-01436: CONNECT BY loop in user data
There is a loop in the user data because John is both the manager and the employee of Jack.
CONNECT BY NOCYCLE and CONNECT_BY_ISCYCLE
With the 10g keyword NOCYCLE, hierarchical queries detect loops and do not generate errors. CONNECT_BY_ISCYCLEpseudo-column is a flag that can be used to detect which row is cycling.
WITH
T
AS
(
SELECT
'JOHN' EMPLOYEE,
'JACK' MANAGER
FROM
DUAL
UNION ALL
SELECT
'JACK' EMPLOYEE,
'JOHN' MANAGER
FROM
DUAL
)
SELECT
SYS_CONNECT_BY_PATH (EMPLOYEE,'/') as EMPLOYEE,
MANAGER,
CONNECT_BY_ISCYCLE
FROM
T
CONNECT BY
NOCYCLE
PRIOR EMPLOYEE = MANAGER;
EMPLOYEE MANAGER CONNECT_BY_ISCYCLE
-------------- ------- ------------------
/JOHN JACK 0
/JOHN/JACK JOHN 1
/JACK JOHN 0
/JACK/JOHN JACK 1
Sub-employee Jack is cycling because he is the child and parent of John. John is also cycling, because he is the child and parent of Jack.
CONNECT BY without PRIOR
A very popular usage of hierarchical query, documented by Vadim Tropashko in his book SQL Design Patterns, is to generate rows.
SELECT
SYS_CONNECT_BY_PATH(DUMMY, '/')
FROM
DUAL
CONNECT BY
LEVEL<4;
SYS_CONNECT_BY_PATH(DUMMY,'/')
--------------------------------
/X
/X/X
/X/X/X
NOTE: According to the official documentation, PRIOR is mandatory.
Oracle Database SQL Language Reference (11.1)
�In a hierarchical query, one expression in the CONNECT BY condition must be qualified by the PRIOR operator�.
The single row of dual is both the parent and the child of itself but no loop is generated. It is a very efficient way to generate rows.
CONNECT_BY_IS_LEAF
The 10g pseudo-column CONNECT_BY_IS_LEAFidentifies rows that do not have descendants.
SELECT
SYS_CONNECT_BY_PATH(ENAME, '/')
FROM
EMP
WHERE
CONNECT_BY_IS_LEAF=1
START WITH
MGR IS NULL
CONNECT BY
PRIOR EMPNO=MGR;
SYS_CONNECT_BY_PATH(ENAME,'/')
-----------------------------------
/KING/JONES/SCOTT/ADAMS
/KING/JONES/FORD/SMITH
/KING/BLAKE/ALLEN
/KING/BLAKE/WARD
/KING/BLAKE/MARTIN
/KING/BLAKE/TURNER
/KING/BLAKE/JAMES
/KING/CLARK/MILLER
Only complete paths from the ancestor where MGR is null to the last descendants are displayed.
WHERE and JOIN
Oracle detects in the where clause if the condition is a join condition or a not-join condition. There is a big difference between a join and a not-join condition in that the former is performed before the building of the hierarchy and the later is performed after the hierarchy.
SELECT
SYS_CONNECT_BY_PATH(EMP.ENAME, '/') P
FROM
EMP,
TABLE
(
SYS.ODCINUMBERLIST(20, 30)
) T
WHERE
EMP.DEPTNO=T.COLUMN_VALUE
CONNECT BY
PRIOR EMP.EMPNO=EMP.MGR
START WITH
MGR IS NULL;
no rows selected
Oracle detects the join and selects the matching rows.
Only rows from the departments 20 and 30 are used to build the hierarchy. No row in the departments 20 and 30 satisfies the condition MGR IS NULL.
SELECT
SYS_CONNECT_BY_PATH(EMP.ENAME, '/') P
FROM
EMP
WHERE
EMP.DEPTNO IN (20, 30)
CONNECT BY
PRIOR EMPNO=MGR
START WITH
MGR IS NULL;
P
---------------------------
/KING/JONES
/KING/JONES/SCOTT
/KING/JONES/SCOTT/ADAMS
/KING/JONES/FORD
/KING/JONES/FORD/SMITH
/KING/BLAKE
/KING/BLAKE/ALLEN
/KING/BLAKE/WARD
/KING/BLAKE/MARTIN
/KING/BLAKE/TURNER
/KING/BLAKE/JAMES
The condition is not a join; therefore, the hierarchy is built with all rows. Later, the condition is applied to the children.
SELECT
P
FROM
(
SELECT
SYS_CONNECT_BY_PATH(EMP.ENAME, '/') P,
DEPTNO
FROM
EMP
CONNECT BY
PRIOR EMPNO=MGR
START WITH
MGR IS NULL
)
WHERE
DEPTNO IN (20, 30);
P
---------------------------
/KING/JONES
/KING/JONES/SCOTT
/KING/JONES/SCOTT/ADAMS
/KING/JONES/FORD
/KING/JONES/FORD/SMITH
/KING/BLAKE
/KING/BLAKE/ALLEN
/KING/BLAKE/WARD
/KING/BLAKE/MARTIN
/KING/BLAKE/TURNER
/KING/BLAKE/JAMES
The same query again appears with a subquery. This time, the hierarchy is built with all rows in the subquery and the condition applies to the main query.
The JOINsyntax allows more flexibility to define what should be processed before the hierarchy and what should be processed after.
SELECT
SYS_CONNECT_BY_PATH(ENAME, '/') P
FROM
EMP
JOIN
DUAL
ON
(
HIREDATE > DATE '1981-06-01'
)
WHERE
JOB='CLERK'
CONNECT BY
PRIOR EMPNO=MGR
START WITH
MGR IS NULL;
P
--------------------
/KING/CLARK/MILLER
The DUAL table is joined to EMP. The ON clause is defined before computing the hierarchy, so the whole tree will contain only employees hired after June 1, 1981. The WHERE clause returns only the clerks. The path from the president to the clerks exclusively contains new employees.