得到当前时间的年份、月份、天、小时等等：
YEAR (current timestamp)
MONTH (current timestamp)
DAY (current timestamp)
HOUR (current timestamp)
MINUTE (current timestamp)
SECOND (current timestamp)
MICROSECOND (current timestamp)
分别得到当时的日期和时间
DATE (current timestamp)
TIME (current timestamp)
关于时间的一些计算：
current date + 1 YEAR
current date + 3 YEARS + 2 MONTHS + 15 DAYS
current time + 5 HOURS - 3 MINUTES + 10 SECONDS
计算两个日期之间有多少天：
days (current date) - days (date(’1999-10-22′))
得到去除毫秒的当前时间:
CURRENT TIMESTAMP - MICROSECOND (current timestamp) MICROSECONDS
将时间转换成字符串：
char(current date)
char(current time)
char(current date + 12 hours)
将字符串转换成时间：
TIMESTAMP (’2002-10-20-12.00.00.000000′)
TIMESTAMP (’2002-10-20 12:00:00′)
DATE (’2002-10-20′)
DATE (’10/20/2002′)
TIME (’12:00:00′)
TIME (’12.00.00′)
