num_rows: get_result vs store_result

问题

The following code returns 0, even though there are 5 entries in the table categories with cat = 1.

$sql = "SELECT name FROM categories WHERE cat = ?";
$stmt = $db->prepare($sql);
$cat = 1;
$stmt->bind_param("i", $cat);
$stmt->execute();
$stmt->get_result();
echo $stmt->num_rows;

However, when I change $stmt->get_result(); to $stmt->store_result(); the output is 5. Why does get_result() not work here?

I found for instance on this answer: https://stackoverflow.com/a/8722329/2311074 that get_result() should also work.

回答1:

Looks like the other answer is wrong and get_result doesn't change the state of a statement (which is quite logical, as you are asking for mysqli_result and therefore supposedly going to work with it from now on).

You see, the way you are using get_result is quite pointless. To make any point of it, you have to assign the result to a variable, which will give you the desired outcome:

$res = $stmt->get_result();
echo $res->num_rows;

Note that the num_rows property is quite useless in general. If you want to know whether your query returned any data or not, just collect the rows into array and then you use this array for the purpose.

$data = $stmt->get_result()->fetch_all();
if ($data) {
    // whatever
} else {
    // oops!
}

And of course you shouldn't use such a query only to count the goods available in a category. For such a purpose a count(*) query have to be used.

来源：https://stackoverflow.com/questions/40192229/num-rows-get-result-vs-store-result

标签

mysqli

prepared-statement

mysql-num-rows