UPDATE mytable t1,
       (
         SELECT col FROM mytable WHERE dt < 10 limit 1
       ) t2
SET t1.col = t2.col
WHERE t1.cd = t2.cd

UPDATE mytable t1
set col = (select col FROM mytable WHERE dt < 10  and col= t1.col limit 1)

------解决方案--------------------
UPDATE mytable t1
set col=t2.col from  
      (
        SELECT col FROM mytable WHERE dt < 10 limit 1
      ) t2
WHERE t1.cd = t2.cd

------解决方案--------------------
UPDATE mytable t1 inner join  
(
 SELECT col FROM mytable WHERE dt < 10 limit 1
 ) t2
on t1.cd = t2.cd
set col=t2.col  



------解决方案--------------------
update daily_reports d1
left join (
   select UserCD,OperationDT,1 idx from
     (SELECT * FROM daily_reports
     where OperationDT < '2010-04-18 02:00:00' and UserCD = '00002073'order by OperationDT desc limit 1)as t
   union
   select UserCD,OperationDT,2 from
     (SELECT * FROM daily_reports
     where OperationDT < '2010-04-18 14:00:00' and UserCD = '00002073' order by OperationDT desc limit 1)as t1
   union
   SELECT UserCD,OperationDT,3 FROM daily_reports where OperationDT = '2010-04-18 14:00:00'  and UserCD = '00002073'
 )d2
on d1.UserCD = d2.UserCD
and d1.OperationDT = d2.OperationDT

left join (
   select UserCD,OperationDT,1 idx from
     (SELECT * FROM daily_reports
     where OperationDT > '2010-04-18 02:00:00' and UserCD = '00002073' order by OperationDT limit 1)as t
   union
     SELECT UserCD,OperationDT,2 FROM daily_reports where OperationDT = '2010-04-18 14:00:00'  and UserCD = '00002073'
   union
   select UserCD,OperationDT,3 from
     (SELECT * FROM daily_reports
     where OperationDT > '2010-04-18 14:00:00' and UserCD = '00002073' order by OperationDT limit 1)as t1

 ) d3
on d2.OperationDT = d3.OperationDT
  and d2.idx = d3.idx

  set d1.CompleteDT=d3.OperationDT