Common table expressions may be recursive. Common table expressions are recursive when the RECURSIVE keyword appears immediately after WITH. A single WITH clause may contain multiple recursive expressions, and may contain both recursive and non-recursive common table expressions.
Recursive common table expressions provide a convenient way to write queries that return relationships to an arbitrary depth. For example, given a table that represents the reporting relationships within a company, you can readily write a query that returns all the employees that report to one particular person.
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.
Recursion provides a much easier means of traversing tables that represent tree or tree-like data structures. The only way to traverse such a structure in a single statement without using recursive expressions is to join the table to itself once for each possible level. For example, if a reporting hierarchy contains at most seven levels, you must join the Employees table to itself seven times. If the company reorganizes and a new management level is introduced, you must rewrite the query.
Recursive common table expressions contain an initial subquery, or seed, and a recursive subquery that during each iteration appends additional rows to the result set. The two parts can be connected only with the operator UNION ALL. The initial subquery is an ordinary non-recursive query and is processed first. The recursive portion contains a reference to the rows added during the previous iteration. Recursion stops automatically whenever an iteration generates no new rows. There is no way to reference rows selected prior to the previous iteration.
The select list of the recursive subquery must match that of the initial subquery in number and data type. If automatic translation of data types cannot be performed, explicitly cast the results of one subquery so that they match those in the other subquery.
For more information about common table expressions, see Introduction to common table expressions.
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