How can PHP developers effectively debug SQL queries that are not returning the expected results, especially when using JOIN or SELF JOIN operations?

When debugging SQL queries that are not returning the expected results, especially when using JOIN or SELF JOIN operations, PHP developers can use tools like var_dump() or print_r() to display the SQL query string before execution. They can also echo out the results of the query to see what data is being returned. Additionally, developers can break down the query into smaller parts and test each part individually to identify where the issue may lie.

// Example PHP code snippet for debugging SQL queries
$sql = "SELECT * FROM table1 JOIN table2 ON table1.id = table2.id";
echo $sql; // Display the SQL query string before execution

$result = mysqli_query($connection, $sql);
if($result){
    while($row = mysqli_fetch_assoc($result)){
        print_r($row); // Output the results of the query
    }
} else {
    echo "Error: " . mysqli_error($connection);
}