聯接子句中的Len函數

Question

Join db_abc 
On a.userid = b.userid 

我想加入所有用戶ID中的長度爲4的所有用戶ID。如何在b.userid上指定一個Len函數，其中用戶ID長度僅爲4個字符？這是在SQL服務器

Answer 1

... 
JOIN db_abc b 
    ON a.userid = b.userid 
     AND LEN(b.userid) = 4 
... 

Answer 2

SELECT * 
FROM a 
INNER JOIN b ON a.userid = b.userid AND LEN(b.userid) = 4