MySQL how to add all of a column together where pid= X - php

Question

Just wondering what would be the best way of adding all the values in a column together where the pid=$pid in a column.
Here's some sample code of what I'm using:
$query = mysql_query("SELECT pid, reputation FROM reputation WHERE pid=\"{$post['pid']}\"");
while($rep = mysql_fetch_assoc($query))
{
echo $rep['reputation'];
}
That works fine, but when more than one row exists where the pid=X I need the reputation column on those rows to add together and output the result.

Answer 1

use GROUP BY
SELECT pid, SUM(reputation) totalReputation
FROM reputation
WHERE pid = 0
GROUP BY pid
in php
$query = mysql_query("SELECT pid, SUM(reputation) totalReputation
FROM reputation
WHERE pid = " .$post['pid'] ."
GROUP BY pid");
and fetch the alias
echo $rep['totalReputation'];
As a sidenote, the query is vulnerable with SQL Injection if the value(s) of the variables came from the outside. Please take a look at the article below to learn how to prevent from it. By using PreparedStatements you can get rid of using single quotes around values.
How to prevent SQL injection in PHP?

Answer 2

$query = mysql_query("SELECT SUM(reputation) as rep FROM reputation WHERE pid=\"{$post['pid']}\"");
while($rep = mysql_fetch_assoc($query))
{
echo $rep['rep'];
}
NOTE: You should not use mysql_* extension since they are now deprecated.