How to create links to original posts, PHP - php

Question

I have a query which returns posts from a database, the query groups the posts by category and the titles are links to the original post. The returned posts are displayed on a categories page.
The trouble is that whilst the post titles are displayed and grouped by category I find that only the first post_id for each post per category is found and only the first post or post with lowest ID in each category is returned by my link. The link brings you to the original post.
Original posts are displayed on a page called “entries.php”
Example:
Post1 id = 1
Post2 id = 2
Post3 id = 3
All the posts above are grouped by category but if I hover over them they all pickup Post1 id=1 for some reason. Is there something I can do to ensure that each id is found when I hover over them?
Thanks for your time!
Query:
$query = "SELECT category, GROUP_CONCAT(DISTINCT title SEPARATOR '<br />') titles, post_id
FROM categories, posts
WHERE categories.category_id = posts.category_id
GROUP BY category";
$result = mysqli_query($dbc, $query)
or die('Error querying database.');
while ($row = mysqli_fetch_array($result)){
echo "<h2>".$row['category']."</h2>";
echo ''.$row['titles'].'';
echo "<hr />";
}

Answer 1

You should GROUP BY post_id, also use ORDER BY yo sort as you whish.

Answer 2

You're missunderstanding SQL's concept of GROUPing. If you GROUP rows on category_id, what you actually are telling MySQL: I want exactly one row for each category_id, regardless of how many rows there exist. In standard SQL you are only allowed to "SELECT category_id" (and aggregate functions like COUNT, MAX, MIN and so on) when using "GROUP BY category_id". MySQL allows you a little more and arbitrarily selects one hit (the first it finds).
What you probably want is ORDER BY category_id, and then do the grouping in your php script like
$prev_cat_id = null;
while ($row = mysqli_fetch_array($result)) {
if ($prev_cat_id != $row['category_id']) {
do stuff for next category
} else {
do stuff for next post in same category
}
}
remember to do cleanup for last category