round() 遵循四舍五入把原值转化为指定小数位数，如：round(1.45,0) = 1;round(1.55,0)=2
floor()向下取整 如：floor(1.45)= 1,floor(1.55) = 1 ,floor(-1.45)= -2 ,floor(-1.55) = -2
ceiling()向上取整 如：ceiling(1.45) = 2,ceiling(1.55)=2,ceiling(-1.45) = -1,ceiling(-1.55)=-1

SQL四舍五入及两种舍入