type 参数可以是下列值：
MICROSECOND
SECOND
MINUTE
HOUR
DAY
WEEK
MONTH
QUARTER
YEAR
SECOND_MICROSECOND
MINUTE_MICROSECOND
MINUTE_SECOND
HOUR_MICROSECOND
HOUR_SECOND
HOUR_MINUTE
DAY_MICROSECOND
DAY_SECOND
DAY_MINUTE
DAY_HOUR
YEAR_MONTH
3、例子：
select now();
#2018-08-13 22:44:28
select DATE_ADD(now(),INTERVAL 2 DAY);
#2018-08-15 22:44:28