Finding Unique Customers Who have AccountNumber Null

SELECT customerid FROM T_TableName

GROUP BY customerid HAVING

SUM   (CASE

WHEN AccountNumber IS NULL THEN 0

ELSE 1

END

)=0

GO

The above query return unique customerid from table T_Tablename if customerid is more than one.

Also it only return if the customerid is having NULL value for all the records. If they have proper account number instead of NULL, the query would not return the customerid.