Question

I have two tables, one that stores incoming messages and one that stores outgoing messages. What I would like is to be able to have a conversations view of the messages so that all incoming and outgoing messages from/to the same user id are grouped and the conversations are ordered by the most recent message (in or out)

Outgoing
----------
user_id
time
message

Incoming
----------
user_id
time
message

What I would like is to display the results such as

-> User A   9:10 pm   Nice ...
<- User A   8:45 pm   Our special is pepperoni!
-> User A   8:00 pm   What's your special dish?

<- User B   9:00 pm   We open at 5
-> User B   6:56 pm   Hello What time to you open?

<- User C   8:43 pm   Thanks!
-> User C   4:00 pm   Loved the pizza today!!

Any idea how to write a query to do this?

EDIT

If user B then texts back in, the result should be:

-> User B   9:15 pm   Ok great!
<- User B   9:00 pm   We open at 5
-> User B   6:56 pm   Hello What time to you open?

-> User A   9:10 pm   Nice ...
<- User A   8:45 pm   Our special is pepperoni!
-> User A   8:00 pm   What's your special dish?

<- User C   8:43 pm   Thanks!
-> User C   4:00 pm   Loved the pizza today!!

Answer 1

You need to UNION the two tables and sort (ORDER BY) accordingly:

SELECT 
    '<-' AS direction,  user_id,  time,  message
FROM
    Outgoing

UNION ALL

SELECT 
    '->',               user_id,  time,  message
FROM
    Incoming

ORDER BY
    user_id ASC,
    time DESC ;

After the additional explanations for the complex ordering:

SELECT
  CASE WHEN m.d = 1 THEN '<-' ELSE '->' END AS direction, 
  m.user_id, m.time, m.message
FROM
    ( SELECT 
        u.user_id, 
        GREATEST( COALESCE(mo.time, mi.time), 
                  COALESCE(mi.time, mo.time) ) AS maxtime
      FROM 
          ( SELECT user_id  FROM Outgoing
          UNION
            SELECT user_id  FROM Incoming 
          ) AS u
        LEFT JOIN
          ( SELECT user_id, MAX(time) AS time  FROM Outgoing  GROUP BY user_id
          ) AS mo
          ON mo.user_id = u.user_id
        LEFT JOIN
          ( SELECT user_id, MAX(time) AS time  FROM Incoming  GROUP BY user_id
          ) AS mi
          ON mi.user_id = u.user_id
    ) AS b    
  JOIN
    ( SELECT 1 AS d, user_id, time, message  FROM Outgoing
    UNION ALL
      SELECT 2 AS d, user_id, time, message  FROM Incoming
    ) AS m
    ON m.user_id = b.user_id
ORDER BY
    b.maxtime ASC,
    m.user_id ASC,
    m.time DESC ;