Birrel Birrel - 6 months ago 35
MySQL Question

PHP, MySQL, PDO - Get result from UPDATE query?

I am updating a row in a table, and trying to return the updated row, as per this SO answer.

My code is the following:

$sql = "SET @update_id := '';
UPDATE testing SET status='1', id=(SELECT @update_id:=id)
WHERE status='0' LIMIT 1;
SELECT @update_id;";

$db->beginTransaction();
try{
$stmt = $db->prepare($sql);
$stmt->execute();

echo count($stmt->fetchAll());

$db->commit();
}catch(Exception $e){
echo $e->getMessage();
exit();
}


But I always get the following error


SQLSTATE[HY000]: General error


Which seems to be due to the
$stmt->fetchAll()
, according to this SO answer. If I take that line out, the row is updated appropriately.

So, how do I run the multi-query statement (multi-statement query!?) using PDO, and obtain the results from the
SELECT
?

EDIT 1

I DO NOT need the count of the rows updated. I need the actual ID of the row.

Table Schema

id | someCol | status
----- | ------- | ------
1 | 123 | 0
2 | 456 | 0
3 | 789 | 0
4 | 012 | 0



  • Look at the table,

  • find the first status=0,

  • update the row,

  • return the id of the row that was updated



The count is of zero interest to me, as the query has
LIMIT 1
hard-coded into it.

The whole point of the line

count($stmt->fetchAll());


Is a pass/fail condition.

if(count ==1){
... do something with the returned id ...
}else{
... do something else ...
}


EDIT 2

Obviously this issue is simple to get around with two separate queries. I would prefer to have this in one single query. Both a preference, as well as an opportunity to learn.

Answer

You need to do the SELECT @update_id as a separate query -- you can't put multiple queries in a single statement. So do:

$sql = "SET @update_id := '';
        UPDATE testing SET status='1', id=(SELECT @update_id:=id)
        WHERE status='0' LIMIT 1";
try{
    $db->beginTransaction();
    $db->query($sql); // no need for prepare/execute since there are no parameters
    $stmt = $db->query("SELECT @update_id");
    $row = $stmt->fetch(PDO::FETCH_ASSOC);
    $id = $row['@update_id'];
    $db->commit();
} catch (Exception $e) {
    echo $e->getMessage();
    $db->rollBack();
    exit();
}