Oracle 19C学习 – 22. WITH AS 语句

什么是WITH AS 语句

WITH AS相当于虚拟视图。 WITH AS也叫做分解子查询或者片段子查询。

定义一个SQL片段， 该片段会被后面的SQL语句用到，可以近似看作一个可用的临时视图。

WITH AS优点

增加了SQL的可读性，如果构造多个片段子查询，结构会更加清晰。
一次构建，多次使用，达到减少读的目标。

SYS_TEMP_XXXX便是运行过程中构造的统计结果临时表。

WITH AS的语法

WITH 临时视图名 AS (SELECT查询)
SELECT .. FROM 临时视图名;

-- 单个表临时视图WITH E AS (SELECT * FROM employees)SELECT * FROM E;-- 连接多个表的临时视图WITH     e as (SELECT * FROM EMPLOYEES),    d as (SELECT * FROM DEPARTMENTS)SELECT e.LAST_NAME, d.department_nameFROM e inner join dON e.department_id = d.department_id;-- 联合多个查询结果WITH     e1 as (SELECT * FROM EMPLOYEES WHERE DEPARTMENT_ID = 100),    e2 as (SELECT * FROM EMPLOYEES WHERE DEPARTMENT_ID = 110)SELECT * from e1UNION ALLSELECT * FROM e2;