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SQL Anywhere 10.0.1 » SQL Anywhere Server - SQL Usage » Common Table Expressions » Recursive common table expressions

Recursive common table expressions Next Page

Selecting hierarchical data

The following query demonstrates how to list the employees by management level. Level 0 represents employees with no managers. Level 1 represents employees who report directly to one of the level 0 managers, level 2 represents employees who report directly to a level 1 manager, and so on.

  manager ( EmployeeID, ManagerID,
            GivenName, Surname, mgmt_level ) AS
( ( SELECT EmployeeID, ManagerID,       -- initial subquery
           GivenName, Surname, 0
    FROM Employees AS e
    WHERE ManagerID = EmployeeID )
  ( SELECT e.EmployeeID, e.ManagerID,   -- recursive subquery
           e.GivenName, e.Surname, m.mgmt_level + 1
    FROM Employees AS e JOIN manager AS m
     ON   e.ManagerID =  m.EmployeeID
      AND e.ManagerID <> e.EmployeeID
      AND m.mgmt_level < 20 ) )
SELECT * FROM manager
ORDER BY mgmt_level, Surname, GivenName;

The condition within the recursive query that restricts the management level to less than 20 is an important precaution. It prevents infinite recursion in the event that the table data contains a cycle.

The max_recursive_iterations option

The option max_recursive_iterations is designed to catch runaway recursive queries. The default value of this option is 100. Recursive queries that exceed this number of levels of recursion terminate, but cause an error.

Although this option may seem to diminish the importance of a stop condition, this is not usually the case. The number of rows selected during each iteration may grow exponentially, seriously impacting database performance before the maximum is reached. Stop conditions within recursive queries provide a means of setting appropriate limits in each situation.