How can I select a value from db - php

Question

Im new in php and sql right now so im confused whats wrong in my codes.(Need to do)cart_tbl (order_id) food_name,special_request, quantity, amount are equal to order_id of my order_tbl. When both order_id of my order_tbl and cart_tbl is same. My output will be the value of that 2 table. This is my code right now.
<?php
$connect = mysqli_connect ("localhost", "root", "" , "db");
if(isset($_POST['order_id'])){
$asd = ($_POST['order_id']);
$sql = "SELECT food_name, special_request, quantity, amount
FROM cart_tbl
WHERE order_id='$asd'";
$result = mysqli_query($connect, $sql);
}
?>
<table class="table table-hover table-bordered">
<thead>
<tr>
<th>Food</th>
<th>Special Request</th>
<th>Quantity</th>
<th>Amount</th>
</tr>
</thead>
<?php
if(mysqli_num_rows($result)>0)
{
while($row = mysqli_fetch_array($result))
{
?>
<tr>
<td><?php echo $row["food_name"];?></td>
<td><?php echo $row["special_request"];?></td>
<td><?php echo $row["quantity"];?></td>
<td><?php echo $row["amount"];?></td>
</tr>
<?php
}
}
?>
</table>

Answer 1

Use INNER JOIN.
SELECT *
FROM cart_tbl
INNER JOIN order_tbl
ON cart_tbl.order_id = order_tbl.order_id
WHERE order_id='$asd'
Or, use NATURAL JOIN if the associated tables have the identical column name order_id and the columns are of same data type.
SELECT *
FROM cart_tbl
NATURAL JOIN order_tbl
WHERE order_id='$asd'