What role does the LEFT JOIN play in the SQL query for combining tables?

The LEFT JOIN in an SQL query is used to combine two tables based on a related column, including all rows from the left table and matching rows from the right table. This means that even if there are no matching rows in the right table, the query will still return all rows from the left table. This can be useful when you want to include all records from one table, regardless of whether there is a match in the other table. 

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

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

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

// SQL query with LEFT JOIN
$sql = "SELECT users.username, orders.order_id FROM users LEFT 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

LEFT JOIN SQL query combining tables PHP databases

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