In what circumstances should tables be linked together in a MySQL query, and how can this be achieved effectively in PHP?

Tables should be linked together in a MySQL query when you need to retrieve data from multiple tables based on a common key or relationship. This can be achieved effectively in PHP by using JOIN clauses in your SQL query to combine the data from different tables based on a specified condition. 

<?php
// Establish a connection to the MySQL database
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "mydatabase";

$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

// SQL query to retrieve data from multiple tables using JOIN
$sql = "SELECT users.username, orders.order_id
        FROM users
        JOIN orders ON users.user_id = orders.user_id";

$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // Output data of each row
    while($row = $result->fetch_assoc()) {
        echo "Username: " . $row["username"]. " - Order ID: " . $row["order_id"]. "<br>";
    }
} else {
    echo "0 results";
}

$conn->close();
?>

Keywords

MySQL tables linked query PHP

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