UPDATE   TmpTbl   
    SET   ma5=
        (
        SELECT   AVERAGE(first)   /*AVERAGE换为AVG*/
            FROM   (
                SELECT   TOP   5  /*这里缺少列*/ 
                    FROM   TmpTbl   
                    WHERE   ID <5   
                    ORDER   BY   ID   DESC
                )
        )
    WHERE   ID=5

------解决方案--------------------
UPDATE   TmpTbl  
SET   ma5=(SELECT   AVERAGE(first)  
          FROM   (SELECT   TOP   5 first  FROM   TmpTbl   WHERE   ID <5   ORDER   BY   ID   DESC))  
WHERE   ID=5

------解决方案--------------------
UPDATE   TmpTbl  
SET   ma5=(
SELECT   AvG(first) 
  FROM   (
SELECT   TOP   5  first  
FROM   TmpTbl  
WHERE   ID <5  
ORDER   BY   ID   DESC)
)  
WHERE   ID=5