猜猜每条SQL是干啥的？

select to_char(trunc(add_months(last_day(to_date('2015-09-02','yyyy-mm-dd')), -1) + 1), 'yyyy-mm-dd') from dual

select trunc(sysdate, 'yyyy') from dual;

select trunc(sysdate,'year')-1+ 0.99999 from dual;

select trunc(sysdate, 'Q') from dual;

select add_months(trunc(sysdate, 'Q'), 3)-1  from dual;

select add_months(trunc(sysdate, 'Q'), 3)-0.00001  from dual;

select trunc(last_day(add_months(sysdate,-1)))+ 0.99999 from dual;

select trunc(last_day(add_months(sysdate,-1)))+ 0.98958 from dual;

select trunc(add_months(last_day(sysdate), -2) + 1)from dual;

select trunc(last_day(add_months(sysdate,-2)))+ 0.99999 from dual;

select trunc(add_months(last_day(sysdate), -1) + 1)from dual;

select trunc(sysdate, 'mm') from dual;

select trunc(sysdate, 'd') + 1 from dual;

select trunc(sysdate, 'd') + 7 from dual;

select SYSDATE from dual;

select TRUNC(SYSDATE - 1)   from dual;

select TRUNC(SYSDATE - 1)+0.99999   from dual;

select extract(year from sysdate) from dual

 
select extract(month from sysdate) from dual

select extract(day from sysdate)　from dual;

select extract(year from date'2019-10-28') year from dual; 
select extract(month from date'2019-10-28') month from dual; 
select extract(day from date'2019-10-28') day from dual; 


select  to_char(sysdate, 'yyyy' )  from dual;

select  to_char(sysdate, 'yyyy' )  from dual; --年


select  to_char(sysdate, 'MM' )  from dual; --月

select  to_char(sysdate, 'dd' )  from dual; --日

select  to_char(sysdate,'Q')  from dual; --季

select  to_char(sysdate,'iw')  from dual; --周--按日历上的那种，每年有52或者53周

select  to_char(sysdate,'hh')  from dual; --hh 小时(12) 
select  to_char(sysdate,'hh24')  from dual; --hh24 小时(24) 
select  to_char(sysdate,'Mi')  from dual; --Mi 分 
select  to_char(sysdate,'ss')  from dual; --ss 秒 
select  to_char(sysdate,'D')  from dual; --D 周中的星期几 
select  to_char(sysdate,'ddd')  from dual; --ddd 年中的第几天 
select  to_char(sysdate,'WW')  from dual; --WW 年中的第几个星期 
select  to_char(sysdate,'W')  from dual; 
--W 该月中第几个星期 --每年的1月1号至1月7号为第一周，以此类推，每年53周 

select to_char(sysdate,'yyyy-MM-dd HH24:mi:ss')  from dual; 

select to_char(sysdate,'yyyy-MM-dd HH24:mm:ss')  from dual; 

select to_char(sysdate,'yy-mm-dd hh24:mi:ss')  from dual;

select to_date('2019-10-25 14:23:31','yyyy-mm-dd,hh24:mi:ss')  from dual 


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

select   sysdate - interval '7' hour  from dual; 

 select   sysdate - interval '7' day  from dual; 

select   sysdate,sysdate - interval '7' month  from dual; 

 select   sysdate,sysdate - interval '7' year  from dual; 

 select   sysdate,sysdate - 2*interval '7' hour  from dual; 

select to_char(sysdate,'yyyy')||'年'||to_char(sysdate,'mm')||'月'||to_char(sysdate,'dd') ||'日' as riqi from dual;

select to_char(last_day(to_date('2019-10-28','YYYY-MM-DD')),'DD') from dual;


select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') as nowTime from dual;   //日期转化为字符串  

select to_char(sysdate,'yyyy')  as nowYear   from dual;   //获取时间的年  

select to_char(sysdate,'mm')    as nowMonth  from dual;   //获取时间的月  

select to_char(sysdate,'dd')    as nowDay    from dual;   //获取时间的日
  
select to_char(sysdate,'hh24')  as nowHour   from dual;   //获取时间的时  

select to_char(sysdate,'mi')    as nowMinute from dual;   //获取时间的分  

select to_char(sysdate,'ss')    as nowSecond from dual;   //获取时间的秒

select to_char(to_date('2019-10-26','yyyy-mm-dd'),'day') from dual;  

select floor(sysdate - to_date('2019-10-01','yyyy-mm-dd')) as days from dual

select to_char(add_months(last_day(sysdate) +1, -2), 'yyyymmdd'),last_day(sysdate) from dual

select add_months(trunc(sysdate,'year'), 12) - trunc(sysdate,'year') from dual
来源：https://blog.csdn.net/qq_37606901/article/details/102779754
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