Double while loop with sql query - php

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.

Answer 2

Essentially you are doing dynamic sql so join would not be possible but adding where clause to second select would make it more efficient. something like - $sql_feat2 = 'SELECT ' + $filter_name + ' FROM wp_wpl_properties where ' + $row2[$filter_feat] + ' is not null';