oracle中least()和greastest()函数的使用，其中还包含一些if...then..elseif的使用

最小值least()函数
情景设定：
从三张表中取出各自的最小时间，比较这三个时间，再取出最小值（其时间可能为null）；

--三个表中各自取出其最小值
select min(v.CHECK_START_TIME) into v_check_v  from t_third_survey_vehicle v where v.claim_id = v_claim_data.claim_id;

select min(v.CHECK_START_TIME) into v_check_c  from t_third_survey_property v where v.claim_id = v_claim_data.claim_id;

select min(v.CHECK_START_TIME) into v_check_p  from T_THIRD_SURVEY_PERSON v where v.claim_id = v_claim_data.claim_id;
--****************************************************************************************************  
--判断其各种情况，并得出最小值      
       if v_check_v is not null and v_check_c is not null and v_check_p is not null then
          v_min_date:=least(v_check_v,v_check_c,v_check_p);

       elsif  v_check_v is null and  v_check_c is null and v_check_p is null then
           v_min_date:=null;         
       end if;

       if v_check_v is null and v_check_c is null then
            v_min_date:=v_check_p;

       elsif v_check_p is null and v_check_c is null then
            v_min_date:=v_check_v;

       elsif v_check_p is null and v_check_v is null then
            v_min_date:=v_check_c;
      end if;

      if v_check_v is null then
          if v_check_c > v_check_p or v_check_c = v_check_p 
            then
              v_min_date:=v_check_p;
          else
               v_min_date:=v_check_c;  
          end if; 
      elsif v_check_c is null then
          if  v_check_v > v_check_p or v_check_v = v_check_p 
            then
              v_min_date:=v_check_p;
            else
             v_min_date:=v_check_v;  
          end if;             
      elsif v_check_p is null then
          if v_check_v > v_check_c or v_check_v = v_check_c then
            v_min_date:=v_check_c;
           else
            v_min_date:=v_check_v;  
          end if;
      end if;

最大值greasest()函数使用方法道理一样