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Oracle "connect by" syntax

Oracle Database Tips by Donald Burleson

Hierarchical Query Pseudo-columns

The new hierarchical query pseudo-columns are valid only in hierarchical queries. The specific hierarchical query pseudo-columns are:

  •  connect_by_iscycle

  • connect_by_isleaf

Additions to the CONNECT BY clause enhance queries of hierarchical data by returning all ancestor-dependent pairs (not just parent-child pairs); a new set of pseudo-columns is provided in Oracle Database 10g:

  • connect_by_iscycle

  •  connect_by_isleaf

  •  connect_by_root

These specify whether or not a given node is a leaf of a hierarchy, a cycle in a hierarchy triggers an error message with information about the rows involved in the cycle, and the CONNECT BY clause is now supported for simple subqueries.

By adding the nocycle parameter in the CONNECT BY condition, we can cause Oracle to return the rows despite the loop. The connect_by_iscycle pseudo-column will show you which rows contain the cycle:

SQL> SELECT ename "Employee", CONNECT_BY_ISCYCLE "Cycle",
   2 LEVEL, SYS_CONNECT_BY_PATH(ename, '/') "Path"
   3 FROM scott.emp
   5 START WITH ename = 'KING'

Employee       CYCLE      LEVEL Path
-------------- -----  --------- ------------------
KING               0          1 /KING
CLARK              1          2 /KING/CLARK
MILLER             0          3 /KING/CLARK/MILLER

Related CONNECT BY Articles:

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