Question

I have a personal message system on my website, which is pretty easy.
But I would like to have an administrator page where all conversations are shown between users and their amount of messages.

So the table looks like (simplified version):

CREATE TABLE pm (
    id        INT(10)     NOT NULL AUTO_INCREMENT,
    from      INT(10)     NOT NULL REFERENCES (usertable),
    to        INT(10)     NOT NULL REFERENCES (usertable),
    message   BLOB        NOT NULL
);

Example:

Say I have some users: Mark, John, Bryan and Kate.

Mark (from) sends 5 messages to John (to) and John (from) sends 3 messages to Mark (to).

Kate (from) sends 2 messages to Bryan (to) and Bryan (from) sends 1 message to Kate (to).

I would like a result set that shows

Mark – John – 8 messages

Kate – Bryan – 3 messages

And this for all users in my table at once.

I’m really stuck on this and I have searched everywhere but have not found a solution.
The difficulty lies in the fact that I want all users listed and I have to cross the "from" and "to" column somehow…

I hope anyone will be able to help.
Thanks in advance.

Answer 1

select from_id, to_id, count(*) count_between
from
(
    select from_id, to_id from pm
    union all
    select to_id, from_id from pm
) combined
where from_id < to_id
group by from_id, to_id

Full sample

CREATE TABLE pm (from_id int,to_id int);
insert pm select 1,2;
insert pm select 1,2;
insert pm select 1,2;
insert pm select 1,2;
insert pm select 1,2;
insert pm select 2,1;
insert pm select 2,1;
insert pm select 2,1;
insert pm select 3,4;
insert pm select 3,4;
insert pm select 4,3;

select from_id, to_id, count(*) count_between
from
(
    select from_id, to_id from pm
    union all
    select to_id, from_id from pm
) combined
where from_id < to_id
group by from_id, to_id

--- results
from_id     to_id       count_between
----------- ----------- -------------
1           2           8
3           4           3

To turn the IDs into names, use the normal to user table or some such. e.g.

select u1.name from_, u2.name to_, count(*) count_between
from
(
    select from_id, to_id from pm
    union all
    select to_id, from_id from pm
) combined
join users u1 on u1.id = combined.from_id
join users u2 on u2.id = combined.to_id
where from_id < to_id
group by u1.name, u2.name