Oracle 知识点积累

1. select  * from dual where to_char(to_date('2018-12-4','yyyy-mm-dd'),'yyyymm')<='201812' 

表示成立

2.select * from dual where to_date('2018-12-4','yyyy-mm-dd')<=to_date('2018-12','yyyy-mm')

表示不成立

3.Select 1 From Dual Connect By Rownum <= 5

取5条数据

4.select count(1) from

(Select 2 From Dual Connect By Rownum <= 3) t2 ,
(Select 1 From Dual Connect By Rownum <= 5)t1

相当于每条记录乘以第二张表的每一条记录：3*5=15

5.select nvl(1,0) from dual

select nvl(x,0) from dual: x=null时候返回0，x！=null时候返回x

6.decode（x，value1，值1，value2，值2，值3）

x=value1时候返回值1

x=value2时候返回值2

x！=value1 且x!=value2时候返回值3