oracle功能强大的with子句

select * from emp ;

	EMPNO	ENAME	JOB	MGR	HIREDATE	SAL	COMM	DEPTNO
1	7369	SMITH	CLERK	7902	17-12月-80	800		20
2	7499	ALLEN	SALESMAN	7698	20-2月 -81	1600	300	30
3	7521	WARD	SALESMAN	7698	22-2月 -81	1250	500	30
4	7566	JONES	MANAGER	7839	02-4月 -81	2975		20
5	7654	MARTIN	SALESMAN	7698	28-9月 -81	1250	1400	30
6	7698	BLAKE	MANAGER	7839	01-5月 -81	2850		30
7	7782	CLARK	MANAGER	7839	09-6月 -81	2450		10
8	7788	SCOTT	ANALYST	7566	19-4月 -87	3000		20
9	7839	KING	PRESIDENT		17-11月-81	5000		10

select * from dept ;

	DEPTNO	DNAME	LOC
1	10	ACCOUNTING	NEW YORK
2	20	RESEARCH	DALLAS
3	30	SALES	CHICAGO
4	40	OPERATIONS	BOSTON

功能强大的WITH子句的用法

WITH
dept_costs AS (
SELECT d.dname, SUM(e.sal) AS dept_total
FROM emp e, dept d
WHERE e.deptno = d.deptno
GROUP BY d.dname ),
avg_cost AS (
SELECT SUM(dept_total)/COUNT(*) AS dept_avg FROM dept_costs )
SELECT * FROM dept_costs
WHERE dept_total <
(SELECT dept_avg FROM avg_cost)
ORDER BY dname ;

官方解释：

The WITH Clause Usage Notes
• It is used only with SELECT statements.
• A query name is visible to all WITH element query blocks (including their subquery blocks)
defined after it and the main query block itself (including its subquery blocks).
• When the query name is the same as an existing table name, the parser searches from the inside
out, the query block name takes precedence over the table name.
• The WITH clause can hold more than one query. Each query is then separated by a comma.