Multiple for-loops from one $result = $conn->query($query) - php

Question

I have imported data into a table and I am now accessing that data with PHP using the code as follows,
<?php
require_once 'connect.php';
$query = "SELECT * FROM JunkData";
$result = $conn->query($query);
if(!$result) die("Fatal Error");
$rows = $result->num_rows;
for ($name = 0; $name < $rows; ++$name)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Name']) . '<br/>';
}
$result->close();
$conn->close();
This works!
I am really just curious why adding a second for-loop does not work, unless I declare $result again?
<?php
require_once 'connect.php';
$query = "SELECT * FROM JunkData";
$result = $conn->query($query);
if(!$result) die("Fatal Error");
$rows = $result->num_rows;
for ($name = 0; $name < $rows; ++$name)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Name']) . '<br/>';
}
for ($number = 0; $number < $rows; ++$number)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Number']) . 'Flag<br/>';
}
$result->close();
$conn->close();
Doesn't work, although 'Flag' is printed an appropriate number of times.
Whereas if I declare $result again.
<?php
require_once 'connect.php';
$query = "SELECT * FROM JunkData";
$result = $conn->query($query);
if(!$result) die("Fatal Error");
$rows = $result->num_rows;
for ($name = 0; $name < $rows; ++$name)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Name']) . '<br/>';
}
$result = $conn->query($query);
for ($number = 0; $number < $rows; ++$number)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Number']) . '<br/>';
}
$result->close();
$conn->close();
The code does work.
I have tried unsetting a few variables with unset($row) etc and I did notice if I remove the line,
$row = $result->fetch_array(MYSQLI_ASSOC);
from the second for loop, it will print the last value in the Number column as many times as the loop will run.
I hope that is understandable. I am wondering what is happening in the code that I need to re-declare $result if I want to run a second for loop against it.

Answer 1

The standard solution I would recommend is to do fetch_all()
Instead of:
$rows = $result->num_rows;
for ($name = 0; $name < $rows; ++$name)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
Do
$rows = $result->fetch_all();
foreach ($rows as $row)
{
...
}
// then you can re-loop same array of $rows
foreach ($rows as $row)
{
...
}

Answer 2

There is an internal pointer when you call fetch_* function.
In your first for loop, you send the pointer to the end of the result set.
So, the next fetch will return nothing.
If you run $result->data_seek(0) you will reset this pointer and can reuse:
for ($name = 0; $name < $rows; ++$name)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Name']) . '<br/>';
}
$result->data_seek(0); //<---- REPLACE HERE
for ($number = 0; $number < $rows; ++$number)
{
$row = $result->fetch_array(MYSQLI_ASSOC);
echo htmlspecialchars($row['Number']) . '<br/>';
}
Of course, usually there is no need to loop the same result set twice, you may need to rethink your logic and loop only once.