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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