PHP and MySQL Select a Single Value

问题

I'd like to know how to select a single value from my MySQL table. The table includes columns username and id amongst others (id is auto-increment and username is unique). Given the username, I want to set a session variable $_SESSION['myid'] equal to the value in the id column that corresponds to the given username. Here's the code that I've already tried:

session_start();
$name = $_GET["username"];
$sql = "SELECT 'id' FROM Users WHERE username='$name'";
$result = mysql_query($sql);
$value = mysql_fetch_object($result);
$_SESSION['myid'] = $value;

So far I'm getting:

Catchable fatal error: Object of class stdClass could not be converted to string.

Casting $value to type string does not fix the problem.

回答1:

1) Don't use quotation in a field name or table name inside the query.

2) After fetching an object you need to access object attributes/properties (in your case id) by attributes/properties name.

It should work
Bonus tips: Use limit 1 for this type of scenario, it will save execution time :)

session_start();
$name = $_GET["username"];
$sql = "SELECT id FROM Users WHERE username='$name' limit 1";
$result = mysql_query($sql);
$value = mysql_fetch_object($result);
$_SESSION['myid'] = $value->id;

One note: please use mysqli_* or PDO since mysql_* deprecated.

回答2:

The mysql_* functions are deprecated and unsafe. The code in your question in vulnerable to injection attacks. It is highly recommended that you use the PDO extension instead, like so:

session_start();

$query = "SELECT 'id' FROM Users WHERE username = :name LIMIT 1";
$statement = $PDO->prepare($query);
$params = array(
    'name' => $_GET["username"]
);
$statement->execute($params);
$user_data = $statement->fetch();

$_SESSION['myid'] = $user_data['id'];

Where $PDO is your PDO object variable. See https://www.php.net/pdo_mysql for more information about PHP and PDO.

For extra help:

Here's a jumpstart on how to connect to your database using PDO:

$database_username = "YOUR_USERNAME";
$database_password = "YOUR_PASSWORD";
$database_info = "mysql:host=localhost;dbname=YOUR_DATABASE_NAME";
try
{
    $PDO = new PDO($database_info, $database_username, $database_password);
}
catch(PDOException $e)
{
    // Handle error here
}

回答3:

You do this by using mysqli_fetch_field method.

session_start();
$link = mysqli_connect("localhost", "my_user", "my_password", "world");
$name = $_GET["username"];
$sql = "SELECT 'id' FROM Users WHERE username='$name' limit 1";
$result = mysqli_query($link, $sql);
if ($result !== false) {
    $value = mysqli_fetch_field($result);
    $_SESSION['myid'] = $value;
}

Note: you can do that by using mysql_fetch_field() method as well, but it will be deprecated in php v5.5

回答4:

Try this

$value = mysql_result($result, 0);

回答5:

When you use mysql_fetch_object, you get an object (of class stdClass) with all fields for the row inside of it.

Use mysql_fetch_field instead of mysql_fetch_object, that will give you the first field of the result set (id in your case). The docs are here

回答6:

mysql_* extension has been deprecated in 2013 and removed completely from PHP in 2018. You have two alternatives PDO or MySQLi.

PDO

The simpler option is PDO which has a neat helper function fetchColumn():

$stmt = $pdo->prepare("SELECT id FROM Users WHERE username=?");
$stmt->execute([ $_GET["username"] ]);
$value = $stmt->fetchColumn();

Proper PDO tutorial

MySQLi

You can do the same with MySQLi, but it is more complicated:

$stmt = $mysqliConn->prepare('SELECT id FROM Users WHERE username=?');
$stmt->bind_param("s", $_GET["username"]);
$stmt->execute();
$data = $stmt->get_result()->fetch_assoc();
$value = $data ? $data['id'] : null;

fetch_assoc() could return NULL if there are no rows returned from the DB, which is why I check with ternary if there was any data returned.

回答7:

It is quite evident that there is only a single id corresponding to a single username because username is unique.

But the actual problem lies in the query itself-

$sql = "SELECT 'id' FROM Users WHERE username='$name'";

O/P

+----+
| id |
+----+
| id |
+----+

i.e. 'id' actually is treated as a string not as the id attribute.

Correct synatx:

$sql = "SELECT `id` FROM Users WHERE username='$name'";

i.e. use grave accent(`) instead of single quote(').

or

$sql = "SELECT id FROM Users WHERE username='$name'";

Complete code

session_start();
$name = $_GET["username"];
$sql = "SELECT `id` FROM Users WHERE username='$name'";
$result = mysql_query($sql);
$row=mysql_fetch_array($result)
$value = $row[0];
$_SESSION['myid'] = $value;

回答8:

try this

session_start();
$name = $_GET["username"];
$sql = "SELECT 'id' FROM Users WHERE username='$name' LIMIT 1 ";
$result = mysql_query($sql) or die(mysql_error());
if($row = mysql_fetch_assoc($result))
{
      $_SESSION['myid'] = $row['id'];
}

回答9:

This is a correct old version.

From documentation correct.

$sql2 = mysql_query("SELECT nombre FROM prov WHERE id_prov = '$array'");
$row2 = mysql_fetch_array($sql2);
$nombre = $row2['nombre'];

Have fun.

mysql_ (Old version)
mysqli_ (New version)

来源：https://stackoverflow.com/questions/59893254/get-id-from-mysql-database-and-load-data-based-on-it