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_name
FROM e inner join d
ON 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 e1
UNION ALL
SELECT * FROM e2;

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