MYSQL: Find all posts between certain users - mysql

Question

I have two tables:
table message (holds the creator of a message and the message)
id - creatorId - msg
and a table message_viewers (tells who can read the message msgId)
msgId - userId
If I create a message as user 1 and send it to user 2 and user 3, the tables will look like this:
tbl_message:
1 - 1 - 'message'
tbl_message_viewers:
1 - 2
1 - 3
What I want to do is to fetch the messages that are between the users x1...xN (any number of users) AND ONLY the messages between them.
(Example if users are 1, 2, and 3, I want the messages where the creator is 1, 2 or 3, and the users are 2,3 for creator = 1, 1 and 3 for creator = 2 and 1, 2 for creator = 3)
I am not interested by messages between 1 and 2, or 2 and 3, or 1 and 3, but only by messages between the 3 people.
I tried different approaches, such as joining the two tables on message id, selecting the messages where creatorId IN (X,Y) and then taking only the rows where userId IN (X, Y) as well. Maybe something about grouping and counting the rows, but I could not figure out a way of doing this that was working.
EDIT: SQL Fiddle here
http://sqlfiddle.com/#!2/963c0/1

Answer 1

I think this might do what you want:
SELECT m.*
FROM message m
INNER JOIN message_viewers mv ON m.id = mv.msgId
WHERE m.creatorId IN (1, 2, 3)
AND mv.userId IN (1, 2, 3)
AND NOT EXISTS (
SELECT 1
FROM message_viewers mv2
WHERE mv2.msgId = mv.msgId
AND mv2.userId NOT IN (1, 2, 3)
)
AND mv.userId != m.creatorId;
The IN's will give the users that created/can see, and the mv.userId != m.creatorId are for excluding the creator from the message_viewers table (like you showed in your requirements).
Edit:
With the requirement of only sending messages between those 3 id's, i came up with the following:
SELECT m.id,m.creatorId,m.message
FROM message m
INNER JOIN message_viewers mv ON m.id = mv.msgId
WHERE m.creatorId IN (1, 2, 3)
AND mv.userId IN (1, 2, 3)
AND mv.userId != m.creatorId
AND NOT EXISTS (
SELECT 1
FROM message_viewers mv2
WHERE mv2.msgId = mv.msgId
AND mv2.userId NOT IN (1, 2, 3)
)
GROUP BY 1,2,3
HAVING COUNT(*) = 2;
sqlfiddle demo

Answer 2

Try this with join and with IN() clause
SELECT * FROM
tbl_message m
JOIN tbl_message_viewers mv (m.id = mv.msgId )
WHERE m.creatorId IN(1,2,3) AND mv.userId IN(1,2,3)

Answer 3

Sounds like you might want the BETWEEN operator:
SELECT * FROM tablename WHERE fieldname BETWEEN 1 AND 10;
-- returns fieldname 1-10
In this case however, BETWEEN is inclusive, so you'll need to specify != those conditions as well:
SELECT * FROM tablename WHERE fieldname BETWEEN 1 AND 10 AND fieldname NOT IN (1, 10)
-- returns fieldname 2-9
http://www.w3schools.com/sql/sql_between.asp

Answer 4

this worked on oracle
first join gets row count for people we are not interested in
second join gets row count for people we are interested in
the in clauses will need to be generated by some sort of dynamic sql
and number_of_people also needs to be generated somehow.
select msgId, count_1, count_2
from message tm
join ( select ty.msgId as ty_msgId,
count(ty.msgId) as count_1
from message_viewers ty
where ty.userId not in (:a,:b,:c)
group by ty.msgId)
on msgId = ty_msgId
join (select tz.msgId as tz_msgId,
count(tz.msgId) as count_2
from message_viewers tz
where tz.userId in (:a,:b,:c)
group by tz.msgId)
on msgId = tz_msgId
where createrId in(:a,:b,:c)
and count_1 = 0
and count_2 = :number_of_people -1;
my sql prefers this
select msgId, count_1, count_2
from message tm
left join ( select ty.msgId as ty_msgId,
count(ty.msgId) as count_1
from message_viewers ty
where ty.userId not in (:a,:b,:c)
group by ty.msgId) as X
on msgId = ty_msgId
left join (select tz.msgId as tz_msgId,
count(tz.msgId) as count_2
from message_viewers tz
where tz.userId in (:a,:b,:c)
group by tz.msgId) as Y
on msgId = tz_msgId
where createrId in(:a,:b,:c)
and (count_1 = 0 or count_1 is null)
and count_2 = :number_of_people -1;