Oracle to_char()函数相关详解

（一）Oracle to_char()函数中的IW,WW 周别显示

1）ww的算法为每年1月1日为第一周开始，date+6为每一周结尾

例如20050101为第一周的第一天，而第一周的最后一天为20050101+6=20050107

　　公式 每周第一天 ：date + 周 * 7 - 7

　　每周最后一天：date + 周 * 7 - 1

2) iw算法为自然周，即星期一到星期日为一周。且每年的第一个星期天为第一周，这样就会把去年的最后几天当成今年的第一周日期！！！

 1 select to_char(date'2015-1-1', 'yyyyww')  from dual;
 2 -- 201501
 3 select to_char(date'2015-1-1', 'yyyyiw')  from dual;
 4 -- 201501
 5 select to_char(date'2015-1-5', 'yyyyww')  from dual;
 6 201501
 7 select to_char(date'2015-1-5', 'yyyyiw')  from dual;
 8 201502
 9 select to_char(date'2014-12-31', 'yyyyww')  from dual;
10 201453
11 select to_char(date'2014-12-31', 'yyyyiw')  from dual;
12 201401 特别注意这里，如果做统计，很容易和真正的201401周数据合并了，其实应该是201501周
13 select to_char(date'2014-12-28', 'yyyyww')  from dual;
14 201452
15 select to_char(date'2014-12-28', 'yyyyiw')  from dual;
16 201452

 推荐使用

　　trunc(date'','iw') --获取每个日期的周一，通过周一日期来排序统计，这样解决跨年的自然周问题

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（二）ORACLE TO_CHAR(SYSDATE,'D')

DDD是该天在一年内的第多少天，d是在一周内第几天，dd是一个月内的

DY ：Day of week abbreviated Mon, Tue, Fri
DAY ：Day of week spelled out Monday, Tuesday, Friday
D ：Day of week (1–7) 1,2,3,4,5,6,7－－注意：每星期的第1天是”星期日“
DD ：Day of month (1–31) 1,2,3,4…31
DDD ：Day of year (1–366) 1,2,3,4…366

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The following are number examples for the to_char function.

to_char(1210.73, '9999.9')	would return '1210.7'
to_char(1210.73, '9,999.99')	would return '1,210.73'
to_char(1210.73, '$9,999.00')	would return '$1,210.73'
to_char(21, '000099')	would return '000021'

The following is a list of valid parameters when the to_char function is used to convert a date to a string. These parameters can be used in many combinations.

Parameter	Explanation
YEAR	Year, spelled out
YYYY	4-digit year
YYY YY Y	Last 3, 2, or 1 digit(s) of year.
IYY IY I	Last 3, 2, or 1 digit(s) of ISO year.
IYYY	4-digit year based on the ISO standard
Q	Quarter of year (1, 2, 3, 4; JAN-MAR = 1).
MM	Month (01-12; JAN = 01).
MON	Abbreviated name of month.
MONTH	Name of month, padded with blanks to length of 9 characters.
RM	Roman numeral month (I-XII; JAN = I).
WW	Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year.
W	Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh.
IW	Week of year (1-52 or 1-53) based on the ISO standard.
D	Day of week (1-7).
DAY	Name of day.
DD	Day of month (1-31).
DDD	Day of year (1-366).
DY	Abbreviated name of day.
J	Julian day; the number of days since January 1, 4712 BC.
HH	Hour of day (1-12).
HH12	Hour of day (1-12).
HH24	Hour of day (0-23).
MI	Minute (0-59).
SS	Second (0-59).
SSSSS	Seconds past midnight (0-86399).
FF	Fractional seconds.

The following are date examples for the to_char function.

to_char(sysdate, 'yyyy/mm/dd');	would return '2003/07/09'
to_char(sysdate, 'Month DD, YYYY');	would return 'July 09, 2003'
to_char(sysdate, 'FMMonth DD, YYYY');	would return 'July 9, 2003'
to_char(sysdate, 'MON DDth, YYYY');	would return 'JUL 09TH, 2003'
to_char(sysdate, 'FMMON DDth, YYYY');	would return 'JUL 9TH, 2003'
to_char(sysdate, 'FMMon ddth, YYYY');	would return 'Jul 9th, 2003'

You will notice that in some examples, the format_mask parameter begins with "FM". This means that zeros and blanks are suppressed. This can be seen in the examples below.

to_char(sysdate, 'FMMonth DD, YYYY');	would return 'July 9, 2003'
to_char(sysdate, 'FMMON DDth, YYYY');	would return 'JUL 9TH, 2003'
to_char(sysdate, 'FMMon ddth, YYYY');	would return 'Jul 9th, 2003'

The zeros have been suppressed so that the day component shows as "9" as opposed to "09".

Oracle函数to_char转化数字型指定小数点位数的用法

to_char，函数功能，就是将数值型或者日期型转化为字符型。

比如最简单的应用：

/*1.0123--->'1.0123'*/ 
Select TO_CHAR(1.0123) FROM DUAL 
/*123--->'123'*/ 
Select TO_CHAR(123) FROM DUAL

 接下来再看看下面：

/*0.123 ---> '.123' */ 
SELEC TO_CHAR(0.123) FROM DUAL

 上面的结果 '.123' 在大多数情况下都不是我们想要的结果，我们想要的应该是 '0.123'。

我们来看一下to_char函数的具体用法：

TO_CHAR ( n [, fmt [, 'nlsparam']] )

该函数将NUMBER类型的n按数值格式fmt转换成VARCHAR2类型的值。'nlsparams'指定由数值格式的元素返回的字符,包括: 

．小数点字符 

．组分隔符 

．本地钱币符号 

．国际钱币符号 

变元的形式为： 

'NLS_NUMERIC_CHARACTERS="dg" NLS_CURRENCY="tcxt" NLS_ISO_CURRENCY=territory' 

其中d为小数点字符,g为组分隔符。 

例 :TO_CHAR (17145,'L099G999','NLS_NUMERIC_CHARACTERS=".," NLS_CURRENCY="NUD"')=NUD017,145

通过上面的了解，再查看fmt的一些格式，我们可以用以下表达式得到'0.123'的值：

/*0.123 ---> ' 0.123' */  
Select TO_CHAR(0.123,'0.999') FROM DUAL 
/*100.12 ---> '######' */  
Select TO_CHAR(100.12,'0.999') FROM DUAL 
/*1.12 ---> ' 1.120' */  
Select TO_CHAR(1.12,'0.999') FROM DUAL

 ' 0.123'是出来了，可是前面又多了一个空格。

对于 100.12 的值却是######，以及'1.12'的值变成了 '1.120'。

我们重新确定一个新的需求：

1、去空格

2、小数点最多4位，最少保留2位。

    1--->'1.00'；1.1--->'1.00'；1.12-->'1.12'；1.1234--->'1.1234'；

    1.12345--->'1.1235'

最终实现如下：

/* 
  FM ：除空格   
  9999999.0099：允许小数点左边最大正数为7位，小数点右边最少2位，最多4位，且在第5位进行四舍五入 
*/  
Select TO_CHAR(123.0233,'FM9999999.0099') FROM DUAL

to_char函数特殊用法 

to_char(sysdate,'d') 每周第几天 
to_char(sysdate,'dd') 每月第几天 
to_char(sysdate,'ddd') 每年第几天 
to_char(sysdate,'ww') 每年第几周 
to_char(sysdate,'mm') 每年第几月 
to_char(sysdate,'q') 每年第几季 
to_char(sysdate,'yyyy') 年 

比如要找某个时间为每周第几天就可以 
SQL> select to_char(to_date('20070101','yyyymmdd'),'d') from dual;