取得当天：
SELECT curdate();
mysql> SELECT curdate(); +------------+ | curdate() | +------------+ | 2013-07-29 | +------------+
取得当前日期：
mysql> select now(); +---------------------+ | now() | +---------------------+ | 2013-07-29 22:10:40 | +---------------------+
取得前一天：
mysql> select date_sub(curdate(),interval 1 day); +------------------------------------+ | date_sub(curdate(),interval 1 day) | +------------------------------------+ | 2013-07-28 | +------------------------------------+
括号中为当天时间的前一天，如果统计前几天就将括号中的’1’改成相应的天数。如果要算月或年，直接将day改为month或year即可
取得前一天的年份：
mysql> SELECT YEAR(DATE_SUB(CURDATE(),INTERVAL 1 DAY)); +------------------------------------------+ | YEAR(DATE_SUB(CURDATE(),INTERVAL 1 DAY)) | +------------------------------------------+ | 2013 | +------------------------------------------+
