to_char与trunc函数 比较区别
出处：http://blog.sina.com.cn/s/blog_6e9fae1301011slj.html

一、to_char函数

Parameter    Explanation
YEAR           Year spelled out
YYYY           4 digits of year
YYY            3 digits of year
YY             2 digits of year
Y              1 digit of year
IYYY           4digits year based on the ISO standard
IYY            3 digits of ISO year
IY             2 digits of ISO year
I              1 digit of ISO year
Q              Quarter of year (1 .. 4)
MM             Month (01 ..12)
MON            Abbreviated name of month
MONTH          Name of month, padded with blanks to length of 9 characters.
RM             Roman numeral month (I .. XII)
WW             Week of year (1-53) where 7 days 1 week (与星期几无关)
W              Week of month (1-5) where 7 days 1 week (与星期几无关)
IW             Week of year (1-52 or 1-53) based on the ISO standard.
               (周一到周日为一周，若1日为周五-周日，则为上年最后一周)
D              Day of week (周日1 .. 周六7)
DY             Abbreviated name of day.
DAY            Name of day
DD             Day of month (1-31)
DDTH           Day of month (1-31)
DDD            Day of year (1-366)
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.
XXXXX          转换为8进制

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'
to_char(21, '999999')      &n