Question

I am trying to create a basic internal messaging system. The only ‘special’ consideration is a user can send a message to multiple recipients and a receiver of a message can Reply To All (if applicable). This is what I currently have:

MESSAGE TABLE
- thread_id
- message_id
- subject
- content
- date_sent
- from (FK to user_id)
- to (FK to user_id(s))

READ TABLE
- message_id (FK)
- recipient (FK to user_id, limit one un-read message per thread)
- read (boolean)

Does this DB design seem sufficient for an internal messaging system, or is it lacking anything / any way to improve it?

Answer 1

It seems like there’s a many-to-many relationship between messages and recipients. I would therefore factor that out into a join table; you may as well put the “read” flag on that join table.

That would give you something like:

MESSAGE TABLE
- thread_id
- message_id
- subject
- content
- date_sent
- from (FK to user_id)


MESSAGE_RECIPIENT TABLE
- message_id (FK)
- recipient (FK to user_id)
- read (boolean)

Your requirement to show only one unread message for a thread is a display issue, and I’d keep it out of the database. In your front-end code, you’d have logic to show the lesser of the actual number of unread messages and 1 when calculating the number of unread messages to show.

To find all recipients for a message (so you can do “reply all”), you need to join to the MESSAGE_RECIPIENT table, as follows:

select u.email_address
from   users u, 
       message m, 
       message_recipient mr
where  u.user_id   = mr.recipient
and    mr.message_id = m.message_id
and    m.message_id  = $whateverYourCurrentMessageIDis

(apologies for the archaic join syntax)