（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
YLast 3, 2, or 1 digit(s) of year. IYY
IY
ILast 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：
Select TO_CHAR(1.0123) FROM DUAL
/*123--->'123'*/
Select TO_CHAR(123) FROM DUAL
接下来再看看下面：
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']] )
．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：
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;