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SQL Anywhere 11.0.1 » SQL Anywhere Server - SQL Usage » Querying and Modifying Data » Common table expressions » Recursive common table expressions


Selecting hierarchical data

Depending on how you write the query, you may want to limit the number of levels of recursion. Limiting the number of levels permits you to return only the top levels of management, for example, but may exclude some employees if the chains of command are longer than you anticipated. Providing no restriction on the number of levels ensures no employees are excluded, but can introduce infinite recursion should the execution require any cycles; for example, if an employee directly or indirectly reports to himself. This situation could arise within a company's management hierarchy if, for example, an employee within the company also sits on the board of directors.

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.

max_recursive_iterations option

The max_recursive_iterations option 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 end, 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.