mysql更新資料的某個欄位，一般這樣寫：

UPDATE mytable SET myfield = 'value' WHERE other_field = 'other_value';

也可以這樣用in指定要更新的記錄：

UPDATE mytable SET myfield = 'value' WHERE other_field in ('other_values');

這裡注意 ‘other_values’ 是一個逗號（，）分隔的字串，如：1,2,3

如果更新多條資料而且每條記錄要更新的值不同，可能很多人會這樣寫：

foreach ($values as $id => $myvalue
 
) {
    $sql = "UPDATE mytable SET myfield = $myvalue WHERE id = $id";
    mysql_query($sql);
}

即是迴圈一條一條的更新記錄。一條記錄update一次，這樣效能很差，也很容易造成阻塞。

那麼能不能一條sql語句實現批量更新呢？mysql並沒有提供直接的方法來實現批量更新，但是可以用點小技巧來實現。

UPDATE mytable
    SET myfield = CASE id
        WHEN 1 THEN 'myvalue1'
        WHEN 2 THEN 'myvalue2'
 

        WHEN 3 THEN 'myvalue3'
    END
WHERE other_field ('other_values')

如果where條件查詢出記錄的id不在CASE範圍內，myfield將被設定為空。

如果更新多個值的話，只需要稍加修改：

UPDATE mytable
    SET myfield1 = CASE id
        WHEN 1 THEN 'myvalue11'
        WHEN 2 THEN 'myvalue12'
        WHEN 3 THEN 'myvalue13'
    END,
    myfield2 = CASE
 
 id
        WHEN 1 THEN 'myvalue21'
        WHEN 2 THEN 'myvalue22'
        WHEN 3 THEN 'myvalue23'
    END
WHERE id IN (1,2,3)

這裡以php為例，構造這兩條mysql語句：

• 更新多條單個欄位為不同值, mysql模式

$ids_values = array(
    1 => 11,
    2 => 22,
    3 => 33,
    4 => 44,
    5 => 55,
    6 => 66,
    7 => 77,
    8 => 88,
);

$ids = implode(',', array_keys($ids_values ));
$sql = "UPDATE mytable SET myfield = CASE id ";
foreach ($ids_values as $id=> $myvalue) {
    $sql .= sprintf("WHEN %d THEN %d ", $id, $myvalue);
}
$sql .= "END WHERE id IN ($ids)";
echo $sql.";<br/>";

輸出：

UPDATE mytable SET myfield = CASE id WHEN 1 THEN 11 WHEN 2 THEN 22 WHEN 3 THEN 33 WHEN 4 THEN 44 WHEN 5 THEN 55 WHEN 6 THEN 66 WHEN 7 THEN 77 WHEN 8 THEN 88 END WHERE id IN (1,2,3,4,5,6,7,8);

• 更新多個欄位為不同值, PDO模式

$data = array(array('id' => 1, 'myfield1val' => 11, 'myfield2val' => 111), array('id' => 2, 'myfield1val' => 22, 'myfield2val' => 222));
$where_in_ids = implode(',', array_map(function($v) {return ":id_" . $v['id'];}, $data));
$update_sql = 'UPDATE mytable SET';
$params = array();

$update_sql .= ' myfield1 = CASE id';
foreach($data as $key => $item) {
    $update_sql .= " WHEN :id_" . $key . " THEN :myfield1val_" . $key . " ";
    $params[":id_" . $key] = $item['id'];
    $params[":myfield1val_" . $key] = $item['myfield1val'];
}
$update_sql .= " END";

$update_sql .= ',myfield2 = CASE id';
foreach($data as $key => $item) {
    $update_sql .= " WHEN :id_" . $key . " THEN :myfield2val_" . $key . " ";
    $params[":id_" . $key] = $item['id'];
    $params[":myfield1va2_" . $key] = $item['myfield2val'];
}
$update_sql .= " END";

$update_sql .= " WHERE id IN (" . $where_in_ids . ")";
echo $update_sql.";<br/>";
var_dump($params);

輸出：

UPDATE mytable SET myfield1 = CASE id WHEN :id_0 THEN :myfield1val_0 WHEN :id_1 THEN :myfield1val_1 END,myfield2 = CASE id WHEN :id_0 THEN :myfield2val_0 WHEN :id_1 THEN :myfield2val_1 END WHERE id IN (:id_1,:id_2);