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.
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 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.
For example, consider the problem of determining which department has the most employees. The Employees table in the SQL Anywhere sample database lists all the employees in a fictional company and specifies in which department each works. The following query lists the department ID codes and the total number of employees in each department.
SELECT DepartmentID, COUNT( * ) AS n FROM Employees GROUP BY DepartmentID;
This query can be used to extract the department with the most employees as follows:
SELECT DepartmentID, n FROM ( SELECT DepartmentID, COUNT( * ) AS n FROM Employees GROUP BY DepartmentID ) AS a WHERE a.n = ( SELECT MAX( n ) FROM ( SELECT DepartmentID, COUNT( * ) AS n FROM Employees GROUP BY DepartmentID ) AS b );
While this statement provides the correct result, it has some disadvantages. The first disadvantage is that the repeated subquery makes this statement less efficient . The second is that this statement provides no clear link between the subqueries.
One way around these problems is to create a view, then use it to re-express the query. This approach avoids the problems mentioned above.
CREATE VIEW CountEmployees( DepartmentID, n ) AS SELECT DepartmentID, COUNT( * ) AS n FROM Employees GROUP BY DepartmentID; SELECT DepartmentID, n FROM CountEmployees WHERE n = ( SELECT MAX( n ) FROM CountEmployees );
The disadvantage of this approach is that some overhead is required, as the database server must update the system tables when creating the view. If the view will be used frequently, this approach is reasonable. However, in cases where the view is used only once within a particular SELECT statement, the preferred method is to instead use a common table expression. For more information about common table expressions, see Using common table expressions.
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.
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