oracle功能强大的with子句

Posted 2020-09-23

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.