mysqli query echo data from two tabels

Question

I have two tables:

bb_users ( user_id, user_name, user_password .... )
bb_topics ( id, time, topic, sender, reciever)

sender and reciever are related to user_id in the bb_users table. For example, sender 450 = user_id 450 = user_name "demouser".

When I export the data from the tables, I want to replace receiver and sender with their user_name .

<?php


$con=mysqli_connect("$DB_HOST","$DB_USER","$DB_PASS","$DB_NAME");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$result = mysqli_query($con,' SELECT * FROM bb_topics LIMIT 3000');

echo "<table border='1'>
<tr>
<th>id</th>
<th>time</th>
<th>topic</th>
<th>sender</th>
<th>receiver</th>
</tr>";

while($row = mysqli_fetch_array($result))
{
echo "<tr>";
echo "<td>" . $row['id'] . "</td>";
echo "<td>"  . $row['time'] . "</td>";
echo "<td>" . $row['topic'] . "</td>";
echo "<td>" . $row['sender'] . "</td>";
echo "<td>" . $row['receiver'] . "</td>";
echo "</tr>";
}
echo "</table>";

mysqli_close($con);
?>

The export looks like:

id   | time        | topic      |  sender    | receiver
1    | 1580471226  | demo topic |    320     |  150 

user_id of sender 320 , his user_name in bb_users = "demotest", receiver = "demo2".

I used LIMIT 3000 because there is a lot of data to be exported. Sometimes I got a PHP fatal error.

How can I convert sender and receiver to their real username, and how to convert time from timestamp to date?

Answer 1

I would advise a more complex SQL Query.

Example: http://sqlfiddle.com/#!9/df5e0e8/14/0

SELECT t.id, FROM_UNIXTIME(t.time, "%Y-%m-%d %H:%i:%s") AS time, t.topic, u1.user_name AS sender, u2.user_name AS receiver 
FROM bb_topics AS t 
LEFT JOIN bb_users AS u1 ON u1.user_id = t.sender 
LEFT JOIN bb_users AS u2 ON u2.user_id = t.receiver;

The output would be:

| id |                time |      topic |   sender | receiver |
|----|---------------------|------------|----------|----------|
|  1 | 2020-01-31 11:47:06 | demo topic | hsimpson | msimpson |

You can also send the Epoch Time to JavaScript if you want.