+----------+-------+
|  Number  | Letter|
+----------+-------+
| 1        |   A   |
| 1        |       |
| 1        |       |
| 2        |       |
| 2        |       |
| 3        |       |
| 3        |   B   |
| 3        |       |
| 3        |   C   |
+----------+-------+

+----------+-------+
|    AC    |  City |
+----------+-------+
| 1        |   A   |
| 1        |   A   |
| 1        |   A   |
| 1        |   A   |
| 2        |       |
| 2        |       |
| 2        |       |
| 2        |       |
| 3        |   B   |
| 3        |   B   |
| 3        |   B   |
+----------+-------+

SELECT Letter
FROM TableA
WHERE TableA.Number = TableB.AC
AND TableA.Number != ""
LIMIT 1

SELECT TOP 1 Letter
FROM TableA
WHERE Letter !='' AND Number=AC

SELECT Letter FROM TableA WHERE Number IN
(SELECT AC FROM TableB WHERE City!='' AND City IS NOT NULL)
AND Letter!='' AND LETTER IS NOT NULL

UPDATE TableB
SET City = Letter
FROM
(
    SELECT Number, Letter,ROW_NUMBER () OVER ( PARTITION BY Number order by number ) AS SortOrder
    FROM TableA
    WHERE Letter IS NOT NULL AND Letter != ''
) AS A
WHERE A.SortOrder = 1 AND TableB.AC = A.number

SELECT TableB.AC, A.Letter
FROM
(
    SELECT Number, Letter,ROW_NUMBER () OVER ( PARTITION BY Number order by number ) AS SortOrder
    FROM TableA
    WHERE Letter IS NOT NULL AND Letter != ''
) AS A
    LEFT OUTER JOIN TableB.AC = A.number
WHERE A.SortOrder = 1