时间相关函数

1. 日期和字符转换函数用法（to_date,to_char）

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_date('2004-05-07 13:23:44','yyyy-mm-dd hh24:mi:ss') from dual;--字符转时间

TO_DATE格式
        Year:     
        yy two digits 两位年                
        yyy three digits 三位年             
        yyyy four digits 四位年    
     
        Month:     
        mm    number     两位月              
        mon    abbreviated 字符集表示    
        month spelled out 字符集表示  
          
        Day:     
        dd    number         当月第几天 
        ddd    number         当年第几天
        dy    abbreviated 当周第几天简写   
        day    spelled out   当周第几天全写
        ddspth spelled out, ordinal twelfth 
             
        Hour:
        hh    two digits 12小时进制
        hh24 two digits 24小时进制 
              
        Minute:
        mi    two digits 60进制                显示值:45
              
        Second:
        ss    two digits 60进制                显示值:25
              
        其它
        Q     digit         季度                  显示值:4
        WW    digit         当年第几周            显示值:44
        W    digit          当月第几周            显示值:1
             
        24小时格式下时间范围为： 0:00:00 - 23:59:59....     
        12小时格式下时间范围为： 1:00:00 - 12:59:59 ....

 2、显示数字的英文

select to_char( to_date(2222,'J'),'Jsp') from dual;--Two Thousand Two Hundred Twenty-Two

3、显示星期

 select to_char(to_date('2019-03-28','yyyy-mm-dd'),'day') from dual; --星期四   

 ALTER SESSION SET NLS_DATE_LANGUAGE='AMERICAN';     

 select to_char(to_date('2019-03-28','yyyy-mm-dd'),'day','NLS_DATE_LANGUAGE = ''SIMPLIFIED CHINESE''') from dual;  --设置日期语言  

Language Name	Language Abbreviation	Default Sort
ALBANIAN	sq	GENERIC_M
AMERICAN	us	binary
AMHARIC	am	GENERIC_M
ARABIC	ar	ARABIC
ARMENIAN	hy	GENERIC_M
ASSAMESE	as	binary
AZERBAIJANI	az	AZERBAIJANI
BANGLA	bn	binary
BELARUSIAN	be	RUSSIAN
BRAZILIAN PORTUGUESE	ptb	WEST_EUROPEAN
BULGARIAN	bg	BULGARIAN
CANADIAN FRENCH	frc	CANADIAN FRENCH
CATALAN	ca	CATALAN
CROATIAN	hr	CROATIAN
CYRILLIC KAZAKH	ckk	GENERIC_M
CYRILLIC SERBIAN	csr	GENERIC_M
CYRILLIC UZBEK	cuz	GENERIC_M
CZECH	cs	CZECH
DANISH	dk	DANISH
DARI	prs	GENERIC_M
DIVEHI	dv	GENERIC_M
DUTCH	nl	DUTCH
EGYPTIAN	eg	ARABIC
ENGLISH	gb	binary
ESTONIAN	et	ESTONIAN
FINNISH	sf	FINNISH
FRENCH	f	FRENCH
GERMAN DIN	din	GERMAN
GERMAN	d	GERMAN
GREEK	el	GREEK
GUJARATI	gu	binary
HEBREW	iw	HEBREW
HINDI	hi	binary
HUNGARIAN	hu	HUNGARIAN
ICELANDIC	is	ICELANDIC
INDONESIAN	in	INDONESIAN
IRISH	ga	binary
ITALIAN	i	WEST_EUROPEAN
JAPANESE	ja	binary
KANNADA	kn	binary
KHMER	km	GENERIC_M
KOREAN	ko	binary
LAO	lo	GENERIC_M
LATIN AMERICAN SPANISH	esa	SPANISH
LATIN BOSNIAN	lbs	GENERIC_M
LATIN SERBIAN	lsr	binary
LATIN UZBEK	luz	GENERIC_M
LATVIAN	lv	LATVIAN
LITHUANIAN	lt	LITHUANIAN
MACEDONIAN	mk	binary
MALAY	ms	MALAY
MALAYALAM	ml	binary
MALTESE	mt	GENERIC_M
MARATHI	mr	binary
MEXICAN SPANISH	esm	WEST_EUROPEAN
NEPALI	ne	GENERIC_M
NORWEGIAN	n	NORWEGIAN
ORIYA	or	binary
PERSIAN	fa	GENERIC_M
POLISH	pl	POLISH
PORTUGUESE	pt	WEST_EUROPEAN
PUNJABI	pa	binary
ROMANIAN	ro	ROMANIAN
RUSSIAN	ru	RUSSIAN
SIMPLIFIED CHINESE	zhs	binary
SINHALA	si	GENERIC_M
SLOVAK	sk	SLOVAK
SLOVENIAN	sl	SLOVENIAN
SPANISH	e	SPANISH
SWAHILI	sw	GENERIC_M
SWEDISH	s	SWEDISH
TAMIL	ta	binary
TELUGU	te	binary
THAI	th	THAI_DICTIONARY
TRADITIONAL CHINESE	zht	binary
TURKISH	tr	TURKISH
UKRAINIAN	uk	UKRAINIAN
VIETNAMESE	vn	VIETNAMESE

 4、计算日期差

select floor(sysdate - to_date('20190327','yyyymmdd')) from dual; 

5、 时间为null的用法

select id, active_date from table1      
   UNION      
   select 1, TO_DATE(null) from dual;  

6、 查找2019-02-01至2019-02-28间除星期一和七的天数     

 select count(*)      
   from ( select rownum-1 rnum      
       from all_objects      
       where rownum <= to_date('2019-02-28','yyyy-mm-dd') - to_date('2019-      
       02-01','yyyy-mm-dd')+1      
      )      
   where to_char( to_date('2019-02-01','yyyy-mm-dd')+rnum-1, 'D' )      
        not in ( '1', '7' )      

7、查找月份 

select months_between(to_date('2019-03-31','yyyy-MM-dd'),to_date（'2019-02-28','yyyy-MM-dd'))"MONTHS" FROM DUAL;      

8、查找下个星期一

select next_day(sysdate,2) from dual;

9、extract()找出日期或间隔值的字段值

  SELECT EXTRACT(YEAR FROM TIMESTAMP '2019-03-27 02:38:40') from dual  

10、处理月份天数不定的办法    

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

11、找出今年的天数

select add_months(trunc(sysdate,'year'), 12) - trunc(sysdate,'year') from dual     

12、闰年的处理方法（判断2月是否是28）

 select to_char( last_day( to_date('02'    | | to_char（sysdate,'yyyy'),'mmyyyy') ), 'dd' ) from dual

13、不同时区的处理 

select to_char( NEW_TIME( sysdate, 'GMT','EST'), 'dd/mm/yyyy hh:mi:ss') ,sysdate      
   from dual;  

 14、字符串转时间戳

select TO_TIMESTAMP_TZ('2019-10-08 00:00:01','YYYY-MM-DD HH24:MI:SS') from dual;