--try
UPDATE db.dbo.dd
SET WCount=a.WCount
FROM db.dbo.aa AS a,db.dbo.dd
WHERE CarNumber like a.CarNumber+'%' And
a.CarType=(CASE WHEN CarType='外' OR CarType='小' THEN '小'
WHEN CarType='大' THEN '大'
WHEN CarType='两' OR CarType='轻' THEN '两'
WHEN CarType='农' THEN '农'
END)
UPDATE db.dbo.dd
SET WCount=a.WCount
FROM db.dbo.aa AS a,db.dbo.dd
WHERE CarNumber like a.CarNumber+'%' And
a.CarType=(CASE WHEN CarType='外' OR CarType='小' THEN '小'
WHEN CarType='大' THEN '大'
WHEN CarType='两' OR CarType='轻' THEN '两'
WHEN CarType='农' THEN '农'
END)
SET
WCount=a.WCount
FROM
db.dbo.aa a,
db.dbo.dd b
WHERE
b.CarNumber like a.CarNumber+'%'
And
a.CarType=(CASE WHEN b.CarType='外' OR b.CarType='小' THEN '小'
WHEN b.CarType='大' THEN '大'
WHEN b.CarType='两' OR b.CarType='轻' THEN '两'
WHEN b.CarType='农' THEN '农'
END)