How to Check if value exists in a MySQL database

Question

Suppose I have this table:

id | name | city
------------------
1  | n1   | c1
2  | n2   | c2
3  | n3   | c3
4  | n4   | c4

I want to check if the value c7 exists under the variable city or not.

If it does, I will do something.
If it doesn't, I will do something else.

Answer 1

using modern MySQLi:

$mysqli = new mysqli(SERVER, DBUSER, DBPASS, DATABASE);

$city = 'c7';
$result = $mysqli->execute_query("SELECT id FROM mytable WHERE city = ? LIMIT 1", [$city]);
if($result->num_rows == 1) {
    // found
}

using legacy mysqli

$mysqli = new mysqli(SERVER, DBUSER, DBPASS, DATABASE);

$city = 'c7';
$stmt = $mysqli->prepare("SELECT id FROM mytable WHERE city = ? LIMIT 1");
$stmt->bind_param("s", $city);
$stmt->execute();
$result = $stmt->get_result();
if($result->num_rows == 1) {
    // found
}

using PDO:

$pdo = new PDO($dsn, $user, $pass, $options);

$city = 'c7';
$stmt = $pdo->prepare("SELECT id FROM mytable WHERE city = ? LIMIT 1");
$stmt->execute([$city]);
if($stmt->rowCount() == 1) {
    // found
}