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1.  取四舍五入的几位小数

                eg:            

                          

round(n+0.{m个0}5, m)

                                       比如， 取两位的话:
                                       round(15.333 + 0.005, 2)

2. 只保留小数

                 eg:

                                        保留两位小数，只舍
                                  zhang@zhang>select trunc(1.2345, 2) from dual;

                                        TRUNC(1.2345,2)
                                        ---------------
                                        1.23

                                  zhang@zhang>select trunc(1.2399, 2) from dual;

                                         TRUNC(1.2399,2)
                                         ---------------
                                        1.23

3.这个比较全了

返回大于或等于x的最大整数:   
        SQL>         select         ceil(23.33)         from         dual;   
    
        CEIL(23.33)   
        -----------   
                          24   
    
        返回等于或小于x的最大整数:   
        SQL>         select         floor(23.33)         from         dual;   
    
        FLOOR(23.33)   
        ------------   
                            23   
    
        返回舍入到小数点右边y位的x值:rcund(x,[y])   
        SQL>         select         round(23.33)         from         dual;   
    
        ROUND(23.33)   
        ------------   
                            23   
    
        返回截尾到y位小数的x值:trunc(x,[y])   
        SQL>         select         trunc(23.33)         from         dual;   
    
        TRUNC(23.33)   
        ------------   
  

                          23