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 1000×1000 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 “*”).