Select multiple rows of one column - php

Question

In MySQL, how can I select the data of one column, only for the rows where the value of the same row, in another column, is session_id (I want all the values, not only the first one)
I have tried this:
SELECT column_name FROM table_name WHERE ID = $session_id
...but it dosen't work. It only selects the first row.
edit, the code I'm using.
<?php
$dn = mysql_query("SELECT IDcontact FROM contacts WHERE ID = '".$_SESSION['id']."'");
if(mysql_num_rows($dn)>0)
{
$dnn = mysql_fetch_array($dn);
$req = mysql_query("select TitreEvent, DescriptionEvent, MomentEvent, image_small from users_event where Confidentialite = 'Public' and ID = " . $dnn['IDcontact']);
while($dnn = mysql_fetch_array($req))
{
?>
So it takes for exemple, the value of the contact/friend (IDcontact), form the database «contacts», where the ID of the logged user is. What I want is to output the event of all the IDcontact, cause actually, it only output the event of the most recent friend added, witch is... the first row of the «contacts» database.

Answer 1

The mysql_fetch_array() function returns only one row from the query's result set. If you want to get all the rows produced by the query, you have to call it in a loop:
while ($row = mysql_fetch_array($dn)) {
// Do stuff with $row...
}
Also, this function is deprecated. You should instead be using either mysqli or PDO to run your queries. See the PHP documentation on choosing an API for more information.
Since you edited your question to show that you're running a second query based on the results of the first one, note that you can do both the IDcontact lookup and get the users_event info in a single query by joining the two tables:
select TitreEvent, DescriptionEvent, MomentEvent, image_small
from users_event
join contacts
on contacts.IDcontact = users_event.ID
where contacts.ID = $session_id
Last but not least, anytime you insert variables (such as your session_id) into a database query, you need to be mindful of SQL injection. If the session ID comes from a parameter that the user can control (e.g. a browser cookie), an attacker could send a malicious session ID that contains SQL code to run arbitrary queries in your database. For safety, you should first create a prepared statement that has a placeholder where the parameter should go:
... where contacts.ID = ?
and then plug in the session_id variable afterward as a "bind parameter". Both mysqli and PDO provide ways to do this: mysqli_stmt_bind_param and PDOStatement::bindParam.