1、获取系统日期：

select sysdate as date1 from dual；

当前时间减去7分钟的时间

select sysdate,sysdate - interval '7' MINUTE FROM dual;

当前时间减去7小时的时间

select sysdate,sysdate - interval '7' HOUR FROM dual;

当前时间减去7天的时间

select sysdate,sysdate - interval '7' DAY FROM dual;

（select (sysdate-7) as date1 from dual）

当前时间减去7月的时间

select sysdate,sysdate - interval '7' MONTH FROM dual;

当前时间减去7年的时间

select sysdate,sysdate - interval '7' YEAR FROM dual;

时间间隔乘以一个数字

select sysdate,sysdate - 8*interval '2' HOUR FROM dual;   

2、日期转换方法：

to_date( '2020-01-01 13:14:20 ', 'yyyy-mm-dd hh24:mi:ss')

3、根据某个字段去重：

--I、Name相同ID最小的记录(推荐用1,2,3),方法3在SQl05时，效率高于1、2

方法1:

Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID<a.ID)

方法2:

select a.* from #T a join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID

方法3:

select * from #T a where ID=(select min(ID) from #T where Name=a.Name)

方法4:

select a.* from #T a join #T b on a.Name=b.Name and a.ID>=b.ID group by a.ID,a.Name,a.Memo having count(1)=1 

方法5:

select * from #T a group by ID,Name,Memo having ID=(select min(ID)from #T where Name=a.Name)

方法6:

select * from #T a where (select count(1) from #T where Name=a.Name and ID<a.ID)=0

方法7:

select * from #T a where ID=(select top 1 ID from #T where Name=a.name order by ID)

方法8:

select * from #T a where ID!>all(select ID from #T where Name=a.Name)

方法9(注:ID为唯一时可用):

select * from #T a where ID in(select min(ID) from #T group by Name)

--SQL2005:

方法10:

select ID,Name,Memo from (select *,min(ID)over(partition by Name) as MinID from #T a)T where ID=MinID

方法11:

select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID) as MinID from #T a)T where MinID=1