记 From DUAL

DUAL：临时表，可以用于构造或者计算数据

Sql Server：不支持该写法！！！！！

Oracle、MySql支持该写法。

例子1：用于无表计算

select 3*4 as result from dual

例子2：将没有关系的结果集构造成一个结果集

select 
(select count(1) from user_info where sex=1) as a1,
(select count(1) from user_info_new where user_tel is not null or user_tel !='') as a2
 from dual

例子3：Insert 前校验数据是否存在。满足where条件才会执行insert

INSERT INTO table (primarykey , name2, name3, ...)

SELECT key, value1, value2, ...

FROM dual

WHERE not exists (select * from table where primarykey = key);