Mysql Query from Query results - php

Question

I am making something like an announcement board that requires readers to acknowledge that they read it, and was wondering if there is a more efficient way of doing this.
I have 3 Tables on MySQL side:
+-----------------+ +-----------------+ +-----------------+
| Announcements | | Acknowledgement | | User |
+-----------------+ +-----------------+ +-----------------+
| announce_id | | ack_id | | user_id |
| announce_msg | | announce_id | | user_name |
| ... | | user_id | | ... |
+-----------------+ +-----------------+ +-----------------+
When a user "reads" the announcement (by clicking a button), Acknowledgment table will be inserted with the Announcement ID and User ID. When a second user "reads" the same announcement, Acknowledgement table will be inserted again with same Announcement ID and the second User ID and so on...
+--------------------------------+
| Acknowledgement |
+--------+-------------+---------+
| ack_id | announce_id | user_id |
+--------+-------------+---------+
| 1 | 1 | 1 |
| 2 | 1 | 4 |
| 3 | 1 | 3 |
| 4 | 3 | 1 |
| 5 | 3 | 6 |
| 6 | 3 | 2 |
+--------+-------------+---------+
Now to the problem. On the front end, when I list all the announcements on a page, I would have to first query for all the announcements. Then, for each announcement, I would have to do another query for all the users that have read this announcement.
$sql = "select * from Announcements";
$result = $pdo->query($sql);
while ($row = $result->fetch())
{
$announce_id = $row['announce_id'];
$announce_msg = $row['annouce_msg'];
$readers = "";
$sql2 = "select u.user_name from Acknowledgement as a INNER JOIN User as u where announcement_id =".$annouce_id;
$result2 = $pdo->query($sql);
while ($row2 = $result2->fetch())
{
$readers .= $row2['user_name'].", ";
}
echo "id:".$annouce_id.", message:".$announce_msg.", Readers:".$readers;
}
So if there 10 announcements on the page, there will be 10 sub-queries for each announcement. What I have now does the job right now... but what if there is 1000 announcements? Then there will be 1000 sub-queries? Sounds like the database will be really hammered. So I'm hoping there is a better way of doing this.
Also, if 1000 people in the user table reads all 1000 announcements, the acknowledgement table will have 1000x1000 entries. seems like the acknowledgement table will become really really long. Will that be a problem as time goes by?
This is a really rough example of what I'm trying to do but it did take me a long time to write all this. If more details is needed let me know.

Answer 1

There is a better way. You can use a single query with group_concat:
select a.*, group_concat(u.user_name separator ', ') as AllUsers
from Announcements a join
Acknowledgement ak
on a.Announce_Id = ak.Announce_Id join
User u
on u.user_ID = ak.User_ID
group by a.announce_id
This uses the MySQL feature of hidden columns to group by only one column (announce_id) but still pull in a bunch of other columns with no aggregations (everything else pulled in by the "*").

Answer 2

If your purpose here is to filter out the announcements that your current user has read, you can do this an entirely different way. Instead of querying for every announcement, and then finding out all the users that have read those announcements and examining those results to find ones that your use has read and trimming them from the displayed list, you can just query in one go for everything a particular user (or list of users) have not yet read.
Change your query to this:
SELECT * FROM Announcements WHERE Announce_id NOT IN (SELECT ANNOUNCE_ID FROM Acknowledgement WHERE User_ID = <INSERT USER ID HERE>)
That should return all Announcement rows that this particular user has not yet acknowledged. If you change that final WHERE clause to be WHERE User_ID IN () then you can specify a list of user IDs.
EDIT: Given the comment you posted above, you could use this query to get all announcements that have been read by no one:
SELECT * FROM Announcements WHERE Announce_id NOT IN (SELECT ANNOUNCE_ID FROM Acknowledgement WHERE User_ID IN (SELECT User_ID FROM User))
The logic for putting together a query to find announcements that haven't been read by someone (if not everyone) is escaping me right this second.
EDIT THE SECOND: Every announcement, and everyone who has and has not read it, requires use of a different kind of join that you've used above, a FULL OUTER JOIN. Unfortunately MySQL doesn't have that feature IIRC, but it can be simulated with a union query
SELECT A.*, ACK.*, U.* FROM Announcements AS A
INNER JOIN Acknowledgement AS ACK ON A.Announce_ID = ACK.Announce_ID
LEFT OUTER JOIN User AS U ON ACK.User_ID = U.User_ID
WHERE U.User_ID IS NOT NULL
UNION ALL
SELECT A.*, ACK.*, U.* FROM Announcements AS A
LEFT OUTER JOIN Acknowledgement AS ACK ON A.Announce_ID = ACK.Announce_ID
RIGHT OUTER JOIN User AS U ON ACK.User_ID = U.User_ID
I think that should do it. No facilities to test at the moment, of course.