What are the potential performance issues with using database queries within nested while loops in PHP?

Using database queries within nested while loops in PHP can lead to performance issues due to the repetitive querying of the database for each iteration. To solve this problem, it is recommended to fetch all the necessary data from the database before entering the nested while loop, and then iterate over the fetched data within the loop.

// Fetch data from the database
$query = "SELECT * FROM table_name";
$result = mysqli_query($connection, $query);

// Check if data is fetched successfully
if ($result) {
    // Iterate over the fetched data
    while ($row = mysqli_fetch_assoc($result)) {
        // Use the data within the loop
        // Nested while loop code here
    }
} else {
    echo "Error fetching data from the database.";
}