PHP and MySQL Select a Single Value [duplicate] - php

Question

This question already has answers here:
Single result from database using mysqli
(6 answers)
Closed 2 years ago.
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.

Answer 1

Don't use quotation in a field name or table name inside the query.
After fetching an object you need to access object attributes/properties (in your case id) by attributes/properties name.
One note: please use mysqli_* or PDO since mysql_* deprecated. Here it is using mysqli:
session_start();
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$link = new mysqli('localhost', 'username', 'password', 'db_name');
$link->set_charset('utf8mb4'); // always set the charset
$name = $_GET["username"];
$stmt = $link->prepare("SELECT id FROM Users WHERE username=? limit 1");
$stmt->bind_param('s', $name);
$stmt->execute();
$result = $stmt->get_result();
$value = $result->fetch_object();
$_SESSION['myid'] = $value->id;
Bonus tips: Use limit 1 for this type of scenario, it will save execution time :)

Answer 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
}

Answer 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

Answer 4

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.
Since PHP 8.1 you can also use fetch_column()
$stmt->execute();
$value = $stmt->get_result()->fetch_column();

Answer 5

Try this
$value = mysql_result($result, 0);

Answer 6

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

Answer 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;

Answer 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'];
}