CODEWITHSUNDEEP
mysql
Varun Jain
Varun Jain

Reputation: 1911

SQL query for most recent messages

I am trying to implement a message system quite similar to facebook . The message table is : 

+--------+----------+--------+-----+----------+
| msg_id | msg_from | msg_to | msg | msg_time |
+--------+----------+--------+-----+----------+

Here msg_from and msg_to contain user ids and the msg_time contains the timestamp of the message . A user's user id can appear in both the to and from column and multiple times for another user . How should I write a SQL query which selects the most recent sent message between two users ? (The message can come from either one) 1 to 2 or 2 to 1 .

Upvotes: 2

Views: 1912

Answers (4)

Jonathan Leffler
Jonathan Leffler

Reputation: 753695

For any complex query like this, use TDQD — Test-Driven Query Design. Devise the answer step-by-step, with the size of the steps controlled by your experience and how well you understand the problem.

Step 1 — Find the time of the most recent message between the given users

Throughout this, I assume that the user IDs are integers; I'm using the values 1000 and 2000.

SELECT MAX(msg_time) AS msg_time
  FROM message
 WHERE ((msg_to = 1000 AND msg_from = 2000) OR
        (msg_to = 2000 AND msg_from = 1000)
       )

Step 2 — Find the record corresponding to the most recent message

SELECT m.*
  FROM message AS m
  JOIN (SELECT MAX(msg_time) AS msg_time
          FROM message
         WHERE ((msg_to = 1000 AND msg_from = 2000) OR
                (msg_to = 2000 AND msg_from = 1000)
               )
       ) AS t
    ON t.msg_time = m.msg_time
 WHERE ((m.msg_to = 1000 AND m.msg_from = 2000) OR
        (m.msg_to = 2000 AND m.msg_from = 1000)
       )

If there happen to be two (or more) messages between these characters with the same latest timestamp, then they'll all be selected; there is at present no basis for choosing between the collisions. If you think that's a problem, you can arrange to find the MAX(msg_id) using the query above (as a sub-query):

SELECT m2.*
  FROM message AS m2
  JOIN (SELECT MAX(m.msg_id) AS msg_id
          FROM message AS m
          JOIN (SELECT MAX(msg_time) AS msg_time
                  FROM message
                 WHERE ((msg_to = 1000 AND msg_from = 2000) OR
                        (msg_to = 2000 AND msg_from = 1000)
                       )
               ) AS t
            ON t.msg_time = m.msg_time
         WHERE ((m.msg_to = 1000 AND m.msg_from = 2000) OR
                (m.msg_to = 2000 AND m.msg_from = 1000)
               )
       ) AS i
    ON i.msg_id = m2.msg_id

Warning: Code not formally tested with any DBMS.

Upvotes: 1

Michael Buen
Michael Buen

Reputation: 39393

Since John Woo clarified that it is not directional, here's my new answer:

select *
from msgsList
where (least(msg_from, msg_to), greatest(msg_from, msg_to), msg_time)       
in 
(
    select 
       least(msg_from, msg_to) as x, greatest(msg_from, msg_to) as y, 
       max(msg_time) as msg_time
    from msgsList 
    group by x, y
);

Output:

| MSG_ID | MSG_FROM | MSG_TO |    MSG |                       MSG_TIME |
------------------------------------------------------------------------
|      1 |        1 |      2 |  hello | January, 23 2010 17:00:00-0800 |
|      5 |        1 |      3 | me too | January, 23 2012 00:15:00-0800 |
|      6 |        3 |      2 |  hello | January, 23 2012 01:12:12-0800 |

For this input:

create table msgsList
(
  msg_id int,
  msg_from int, 
  msg_to int,
  msg varchar(10),
  msg_time datetime
);

insert into msgslist VALUES

(1, 1, 2, 'hello', '2010-01-23 17:00:00'),      -- shown
(2, 2, 1, 'world', '2010-01-23 16:00:00'),

(3, 3, 1, 'i am alive', '2011-01-23 00:00:00'),
(4, 3, 1, 'really', '2011-01-22 23:15:00'),
(5, 1, 3, 'me too', '2012-01-23 00:15:00'),     -- shown

(6, 3, 2, 'hello', '2012-01-23 01:12:12');      -- shown

SQLFiddle Demo

If ANSI SQL is your cup of tea, here's the way to do it: http://sqlfiddle.com/#!2/0a575/19

select *
from msgsList z
where exists
(
    select null
    from msgsList
    where 
      least(z.msg_from, z.msg_to) = least(msg_from, msg_to)
      and greatest(z.msg_from, z.msg_to) = greatest(msg_from, msg_to)
    group by least(msg_from, msg_to), greatest(msg_from, msg_to)
    having max(msg_time) = z.msg_time  
) ;

Upvotes: 4

Michael Buen
Michael Buen

Reputation: 39393

Could it be this simple? http://www.sqlfiddle.com/#!2/50f9f/1

set @User1 := 'John';
set @User2 := 'Paul';


select *
from
(
  select *
  from messages 
  where msg_from = @User1 and msg_to = @User2
  order by msg_time desc
  limit 1
) as x
union
select *
from
(
  select *
  from messages 
  where msg_from = @User2 and msg_to = @User1
  order by msg_time desc
  limit 1
) as x
order by msg_time desc

Output:

| MSG_ID | MSG_FROM | MSG_TO |         MSG |                      MSG_TIME |
----------------------------------------------------------------------------
|      2 |     Paul |   John | Hey Johnny! | August, 20 2012 00:00:00-0700 |
|      1 |     John |   Paul | Hey Paulie! | August, 19 2012 00:00:00-0700 |

Could be a lot simpler if only MySQL supported windowing function: http://www.sqlfiddle.com/#!1/e4781/8

with recent_message as
(
select *, rank() over(partition by msg_from, msg_to order by msg_time desc) as r
from messages
)
select * 
from recent_message 
where r = 1 
    and 
    (
      (msg_from = 'John' and msg_to = 'Paul') 
      or
      (msg_from = 'Paul' and msg_to = 'John')
    )
order by msg_time desc;

Upvotes: 2

Pieter Bos
Pieter Bos

Reputation: 1574

After giving it some thought, I came up with this:

SELECT min_user AS min(msg_from, msg_to), max_user AS max(msg_from, msg_to), 
max(msg_date) FROM msg GROUP BY min_user, max_user

I'm still not quite sure how to get the additional data from the message, but I'll give it some thought.

Upvotes: 0

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