C:\Documents and Settings\db2admin>db2 select * from cl

ACC_NUM       AMT
------------- -----------
AB112                  27
AB113                  47
AB114                  87
AB115                  37
AB116                  97
AB117                  67
AB118                  87
AB119                  77

  8 record(s) selected.

C:\Documents and Settings\db2admin>db2 select * from tm

ACC_NUM       AMT
------------- -----------
AB119                  70
AB116                  40
AB113                  20

  3 record(s) selected.

C:\Documents and Settings\db2admin>db2 update cl source set amt =
 select amt from tm target where source.acc_num=target.acc_num)

DB21034E  The command was processed as an SQL statement because
 it was not avalid Command Line Processor command.  During SQL 
processing it returned:SQL0407N  Assignment of a NULL value to 
a NOT NULL column "TBSPACEID=4,TABLEID=530, COLNO=1" is 
not allowed.  SQLSTATE=23502

UPDATE CL SOURCE SET
        AMT = (
        
                 SELECT AMT FROM TM TARGET
                 WHERE SOURCE.ACC_NUM=TARGET.ACC_NUM
        
                  );

 WHERE EXISTS (
                 SELECT AMT FROM TM TARGET
                 WHERE SOURCE.ACC_NUM=TARGET.ACC_NUM
              )

MERGE INTO
      cl target
USING tm source
  ON  source.acc_num = target.acc_num
WHEN MATCHED THEN
      UPDATE
      SET amt = source.amt
;