在PHP中，为什么mysqli_num_rows对于一个有0条返回记录的查询不返回整数？[英] In PHP, why doesn't mysqli_num_rows return an integer for a query with 0 returned rows?

问题描述

I am having a strange issue with mysqli_num_rows. Searching for this issue, I've only found people having issues where NULL is returned no matter what. I also checked the official documentation for the function, and it says it returns an integer of the number of rows returned by the query. Whenever my query returns 1 row (it never should return more), it behaves as I expect. When the query returns 0 rows, I expect the function to return 0, but it returns NULL. Why doesn't it return 0?

I know that my database connection is good and my query works correctly, because when I look for a record that's in the database, I get an integer back. I just can't figure out why this is returning NULL rather than 0.

function getArtistID($name) {
    global $conn; 
    $query = "SELECT artist_id FROM artist WHERE artist_name LIKE '${name}'";
    $result = mysqli_query($conn, $query);
    if ($result->num_rows) {
        $row = mysqli_fetch_assoc($result);
        return $row['artist_id'];
    } else {
        return 0;
    }
}

推荐答案

Here's some code that I used to reproduce a case where num_rows seems to be NULL:

<?php
error_reporting(E_ALL);
$conn = new mysqli('127.0.0.1', 'root', null, 'test');
$sql = "SELECT * FROM dual";
$result = $conn->query($sql);
if ($result === false) {
  print "Error: {$conn->error}\n";
}
$n = $result->num_rows;
echo "Dump the num_rows property: ";
var_dump($n);

Output:

Error: No tables used

Notice: Trying to get property of non-object in /Users/bkarwin/Documents/SO/my.php on line 14
Dump the num_rows property: NULL

The notice is because it's invalid to access an object-oriented property of a variable that is not an object. This is a frequent source of confusion for PHP developers, and it's a byproduct of the fact that PHP is a loosely typed language, and functions like query() can return either a result object, or a boolean scalar.

The query() function actually returned a false as $result because of some error. In my code, I checked for this error, and you didn't.

When you run mysqli::query() or mysqli::prepare() or mysqli_stmt::execute(), you must check for error conditions every time.

Something about your query caused an error. It's up to you to check for the error and report it.

Update: I edited some text above to make the explanation better, but it might make some comments below seem out of place.

其他推荐答案

I just can't figure out why this is returning NULL rather than 0.

We can only guess without seeing the log output; but, it is likely the return value is null because it raised an error instead.

You need to ensure that errors are handled when calling a function, before attempting to use the return value.