Mysqli query multiple tables - php

Question

I'm trying to make a query that then displays certain results based on previous queries
The idea is that when someone logs into the page, it gets the session username and and saves it to a variable, from there the first query selects a row based on the session username, gets that value and does the same in the second query but on a different table this time getting the row based on the result from query 1 and query 3 is same as two and then its meant to echo it out
here's the code
$con = mysqli_connect("localhost","root","","boats4u");
$search = $_SESSION['myusername'];
if(mysqli_connect_errno())
{
echo "Failed to connect to database". mysqli_connect_error();
}
$pre_res = mysqli_query($con,"SELECT ownerNo FROM boatowner WHERE email ='$search'");
$pre_res = $pre_res -> fetch_assoc();
$result = mysqli_query($con,"SELECT boatNo FROM boatforrent WHERE ownerNo ='$pre_res'");
$result = $result -> fetch_assoc();
$result2 = mysqli_query($con,"SELECT * FROM boatviewing WHERE boatNo = '$result'");
echo "<table border='1'>
<tr>
<th>Client No</th>
<th>Boat No</th>
<th>View Date</th>
<th>Comments</th>
</tr>";
while ($row = mysqli_fetch_array($result2))
{
echo "<tr>";
echo "<td>". $row['clientNo']."</td>";
echo "<td>". $row['boatNo']."</td>";
echo "<td>". $row['viewDate']."</td>";
echo "<td>". $row['comment']."</td>";
}
echo "</table>";
?>
this is what displays
Notice: Array to string conversion in
E:\Download\Xampp\htdocs\owner.php on line 29
If I remove the first query then it no errors but obviously the search doesn't work then
any help appreciated

Answer 1

You should do one query and also parametize the search parameter. Something along the lines like:
$stmt = $con->prepare('
SELECT boatviewing.*
FROM boatowner owner
LEFT JOIN boatforrent ON boatforrent.ownerNo = owner.ownerNo
LEFT JOIN boatviewing ON boatviewing.boatNo = boatforrent.boatNo
WHERE owner.email = ?
');
$stmt->bind_param("s", $search);
$stmt->execute();
$result = $stmt->get_result();
Such code is normally more robust against SQL injection and it's also easier in case you change your database layout.
Next to that you actually run one query instead of three which allows the database to optimize data-retrieval and keeps roundtrips between the PHP script and the database server low.