Application Programming and SQL Guide - Kmlinux

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Using common table expressions with CREATE VIEW
You can use common table expressions before a fullselect in a CREATE VIEW
statement. The common table expression must be placed immediately inside of the
statement. This is useful if you need to use the results of a common table
expression in more than one query.
Example: Using a WITH clause in a CREATE VIEW statement: The following
statement finds the departments that have a greater than average total pay and
saves the results as the view RICH_DEPT:
CREATE VIEW RICH_DEPT (deptno) AS
WITH DTOTAL (deptno, totalpay) AS
(SELECT deptno, sum(salary+bonus)
FROM DSN8810.EMP
GROUP BY deptno)
SELECT deptno
FROM DTOTAL
WHERE totalpay > (SELECT AVG(totalpay)
FROM DTOTAL);
The fullselect in the previous example uses the result table for DTOTAL to find the
departments that have a greater than average total pay. The result table is saved as
the RICH_DEPT view and looks similar to the following results:
DEPTNO
======
A00
D11
D21
Using common table expressions when you use INSERT
You can use common table expressions before a fullselect in an INSERT statement.
The common table expression must be placed immediately inside of the statement.
Example: Using a WITH clause in an INSERT statement: The following example
illustrates the use of a common table expression in an INSERT statement.
INSERT INTO vital_mgr (mgrno) AS
WITH VITALDEPT (deptno, se_count) AS
(SELECT deptno, count(*)
FROM DSN8810.EMP
WHERE job = ’senior engineer’
GROUP BY deptno)
SELECT d.manager
FROM DSN8810.DEPT d, VITALDEPT s
WHERE d.deptno = s.deptno
AND s.se_count > (SELECT AVG(se_count)
FROM VITALDEPT);
The fullselect in the previous example uses the result table for VITALDEPT to find
the manager’s number for departments that have a greater than average number of
senior engineers. The manager’s number is then inserted into the vital_mgr table.
Using recursive SQL
You can use common table expressions to create recursive SQL. If a fullselect of a
common table expression contains a reference to itself in a FROM clause, the
common table expression is a recursive common table expression. Queries that use
recursion are useful in applications like bill of materials applications, network
planning applications, and reservation systems.
Recursive common table expressions must follow these rules:
Chapter 1. Retrieving data 15