日期类型的操作 year() 取一个date数据类型的年份 where year(date) = 1997 between purchase_date between Prices.start_date and Prices.end_date left/right 选择一个date数据“2020-19-12”的左七位数字作为month的标记。 left(trans_date, 7) as month datediff 计算两个时间之差（前者减后者） join XXX on datediff(a.visited_on, b.visited_on) between 0 and 6