使用sql查询双循环

Question

I created a code that is working fine but I'm not sure if its 'legit'. I am using a sql query in a while loop from another sql query, that means that the (second) sql query is repeated the amount of rows the first query returns.

Can anyone tell me if I can use this or its just one complete mess?

the code:

$conn = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

$sql_feat = 'SELECT * FROM wp_wpl_dbst WHERE category = 105';
$result_feat = mysqli_query($conn, $sql_feat);

if (mysqli_num_rows($result_feat) > 0) {

    while($row = mysqli_fetch_assoc($result_feat)) {

        $filter_feat = $row["table_column"];
        $filter_name = $row["name"];

        $sql_feat2 = 'SELECT * FROM wp_wpl_properties';
        $result_feat2 = mysqli_query($conn, $sql_feat2);

        if (mysqli_num_rows($result_feat2) > 0) {

            while($row2 = mysqli_fetch_assoc($result_feat2)) {

                if (!empty($row2[$filter_feat])) {
                    echo $filter_name;
                    echo "<br>";                                         
                } 
            }
        }                                                          
    }
}

mysqli_close($conn); 

Answer 1

First, you should always check the result of mysqli_query before assuming it represents a proper query result:

// you should check if this value is FALSE before using it
$result_feat = mysqli_query($conn, $sql_feat);

// and this too
$result_feat2 = mysqli_query($conn, $sql_feat2);

As for your code running a query inside the loop, it is pretty much always discouraged unless there is no other option. You should explore the possibility of a JOIN before running any queries inside a loop. Cases may exist where you have no other option, but in your code, I don't see any point at all to run this query over and over:

$sql_feat2 = 'SELECT * FROM wp_wpl_properties';

If it made reference to the value from $row, then maybe. But it doesn't.