mysql - INSERT SELECT ON DUPLICATE が更新されない

Question

短い

1. SUM列にTABLE_A基づいてCRITERIA X挿入したいTABLE_B.total_x
2. SUM列にTABLE_A基づいてCRITERIA Y挿入したいTABLE_B.total_y
3. 問題: ステップ 2 が更新されないTABLE_B.total_y

長いです

TABLE_A: データ

| year | month | type | total |
---------------------------------------
| 2013 | 11    | down | 100   |
| 2013 | 11    | down | 50    |
| 2013 | 11    | up   | 60    |
| 2013 | 10    | down | 200   |
| 2013 | 10    | up   | 15    |
| 2013 | 10    | up   | 9     |

TABLE_B: 構造

CREATE TABLE `TABLE_B` (
    `year` INT(4) NULL DEFAULT NULL,
    `month` INT(2) UNSIGNED ZEROFILL NULL DEFAULT NULL,
    `total_x` INT(10) NULL DEFAULT NULL,
    `total_y` INT(10) NULL DEFAULT NULL,
    UNIQUE INDEX `unique` (`year`, `month`)
)

SQL: CRITERIA_X

INSERT INTO TABLE_B (
 `year`, `month`, `total_x`
)
SELECT 
  t.`year`, t.`month`,
  SUM(t.`total`) as total_x
FROM TABLE_A t
WHERE
  t.`type` = 'down'
GROUP BY
  t.`year`, t.`month`
 ON DUPLICATE KEY UPDATE
  `total_x` = total_x
;

SQL: CRITERIA_Y

INSERT INTO TABLE_B (
 `year`, `month`, `total_y`
)
SELECT 
  t.`year`, t.`month`,
  SUM(t.`total`) as total_y
FROM TABLE_A t
WHERE
  t.`type` = 'up'
GROUP BY
  t.`year`, t.`month`
 ON DUPLICATE KEY UPDATE
  `total_y` = total_y
;

2 番目の SQL (CRITERIA_Y) が期待どおりに更新されませんtotal_y。なぜ？