PHP和MySQL - >显示数据

I have successfully accomplished my first step (link to my G+ post, but you don't have to read it. Everything is here). Now there are some complications. This is demo table:

ID | userid | Age | Name
---------------------------
 1 |  1     |  30 | John
 2 |  1     |  31 | Mike
 3 |  1     |  30 | Whoever
 4 |  2     |  32 | Jack
 5 |  2     |  31 | Alice
 6 |  3     |  30 | Kurt

I would like to display only rows with userid = 1 + persons with the same age have to be in the same row:

userid      | Age | Name
---------------------------------
 1 (hidden) |  30 | John, Whoever
 1 (hidden) |  31 | Mike

In my database I'm using subject and grade instead of age and name. This is how my MySQL query looks right now:

SELECT
p.subject as 'subject'
GROUP_CONCAT(grade) as grades
FROM grades p
GROUP BY p.subject

This displays data for every userid. Userid isn't always 1 so I can't just say if($userid == 1){/*code*/}else{/*code*/} or anything similar.

How can I accomplish this?

Some PHP code I'm using to display data:

<?php
$result = $dbc->query("
SELECT
p.subject as 'subject',
id, finished, date,
GROUP_CONCAT(grade) as names
FROM grades p
GROUP BY p.subject
");
?>
<table>
<tr>
    <th>Subject</th>
    <th>Grade</th>
</tr>
<?php while($row = $result->fetch_assoc()){
    $names = split(",",$row["names"]);
?>
    <tr>
        <td><?php echo $row["subject"] ?> </td>
        <td><?php foreach( $names as $name){echo $name . ' ' ; } ?>
    </tr>

<?php } ?>
</table>?

Ok I have been playing around with this and I have got it

Looks like you want to group by both user id and age so that you can perform aggregates on data which both have a specific userid and age.

I have included the code to create the table and insert the data incase anyone reading the question wanted to have a quick play.

create table grades
(
id int(2),
userid int(2),
age int(2),
name varchar(20),
)

insert into grades values
(1,1,30,'John'),
(2,1,31,'Mike'),
(3,1,30,'Whoever'),
(4,2,32,'Jack'),
(5,2,31,'Alice'),
(6,3,30,'Kurt');

SELECT
userid, age, GROUP_CONCAT(name)
FROM grades
GROUP BY userid, age

http://sqlfiddle.com/#!9/7d9ac/3

SELECT Age, GROUP_CONCAT(Name)
FROM grades
WHERE userid=1
GROUP BY Age