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.