Returning first result where CASE result = X? - mysql

Question

I'm trying to return to write a query that only returns first result where someid (result of the first case)=X - and ignoring subsequent results where someid=X. How would I alter the following query to achieve that?
Here's what I'm currently using:
SELECT DISTINCT CASE WHEN $userid != senderid THEN senderid ELSE GROUP_CONCAT(receivers.id SEPARATOR ', ') END someid,
CASE WHEN $userid != senderid THEN senders.username ELSE GROUP_CONCAT(receivers.username SEPARATOR ', ') END somename,
messages.body,
messages.time
FROM messages
LEFT JOIN messages_recipients AS recipients ON messages.id = recipients.messageid
LEFT JOIN users AS senders ON messages.senderid = senders.id
LEFT JOIN users AS receivers ON recipients.userid = receivers.id
WHERE recipients.userid = $userid
OR messages.senderid = $userid
GROUP BY messages.id
ORDER BY messages.time DESC

Answer 1

Based on the information from your comments, I have taken another stab at this.
I'm very hands-on so although I can conceptualize what will happen, I didn't know for sure till I started tinkering around.
So, let's start with the data. Based on what we talked about, I created three tables:
users
id user_name
1 Walker
2 John
3 Kate
messages
id senderid body time
1 1 ignored 1 2010-04-01 00:00:00.000
2 1 ignored 2 2010-04-02 00:00:00.000
3 3 ignored 3 2010-04-03 00:00:00.000
4 1 msg A to john and kate 2010-04-10 00:00:00.000
5 3 msg b from kate to walker and john 2010-04-11 00:00:00.000
*messages_recipients*
id messageid userid
1 1 2
2 1 3
3 2 2
4 3 1
5 4 2
6 4 3
7 5 1
8 5 2
The data is tailored in such a way that you (Walker) have both sent and received messages through the month of April with John and Kate.
You can see a list of these messages by running the following sql statement:
SELECT
u2.user_name AS Sender,
u1.user_name AS Receiver,
m.body,
m.time
FROM
messages m
JOIN
messages_recipients mr ON m.id = mr.messageid
JOIN
users u1 ON mr.userid = u1.id
JOIN
users u2 ON m.senderid = u2.id
ORDER BY
time DESC
Now, that we have the test scenario, the tricky part: returning the most recently communicate message between you (Walker) and John and Kate. I've got a rather long SQL statement and, admittedly, I am not the best at creating these, but I think this will still work:
BEGIN
DECLARE #UserId INT = 1
--A. Main Query
SELECT
CASE
WHEN mtemp.senderid = 1 --#UserId
THEN
CONCAT('Message To: ', receivers.user_name)
ELSE
CONCAT('Message From: ' , senders.user_name)
END AS MessageType,
mtemp.body,
mtemp.time
FROM
messages mtemp
INNER JOIN users senders ON
mtemp.senderid = senders.id
INNER JOIN
(
--B. Inner Query determining most recent message (based on time)
-- between #UserID and the person #UserID
-- Communicated with (either as sender or receiver)
select userid,max(maxtime) as maxmaxtime from
(
--C.1. First part of Union Query Aggregating sent/received messages on passed #UserId
SELECT
m2.body,
kk.*
FROM
`messages` m2 INNER JOIN
(
SELECT DISTINCT
userid,
MAX(m.time) AS MaxTime
FROM
messages m INNER JOIN
messages_recipients mr ON m.id = mr.messageid AND
m.senderid = 1 --#UserId
GROUP BY
mr.userid
) kk on m2.time = kk.MaxTime and m2.senderid = 1 --#UserId
UNION
--C.2. Second part of Union Query Aggregating sent/received messages on passed #UserId
SELECT
m1.body,
jj.*
FROM
`messages` m1 INNER JOIN
----C.2a. Inner most query of users users who sent message to userid
(SELECT DISTINCT
senderid as userid,
MAX(m.time) AS MaxTime
FROM
messages m INNER JOIN
messages_recipients mr ON m.id = mr.messageid AND
mr.userid = 1 --#UserId
GROUP BY
m.senderid) jj on m1.time = jj.MaxTime and m1.senderid = jj.userid
) MaximumUserTime
group by
MaximumUserTime.userid
) AggregatedData on mtemp.time = AggregatedData.maxmaxtime
INNER JOIN users receivers on AggregatedData.userid = receivers.id
ORDER BY `time` DESC
END
To test in phpMyAdmin, you'll have to remove the comments and the begin/end declare statements as well. I just wanted to post this as if it would look in a procedure.
The query assumes that you won't both send and receive a different message from the same user at the exact same time; the odds of that happening seem very small so I'm hoping may this will work.
When I run this query I get the following results:
MessageType body time
Message From: Kate msg b from kate to walker and john 2010-04-11 00:00:00.000
Message To: John msg A to john and kate 2010-04-10 00:00:00.000
That's the most recent communications concerning Walker among all those users who have communicated with Walker.
Hope that helps.

Answer 2

A sub-query may work in this case (though the performance of it may not make it a great option):
SELECT DISTINCT CASE WHEN $userid != senderid THEN senderid ELSE GROUP_CONCAT(receivers.id SEPARATOR ', ') END someid,
CASE WHEN $userid != senderid THEN senders.username ELSE GROUP_CONCAT(receivers.username SEPARATOR ', ') END somename,
messages.body,
messages.time
FROM messages
INNER JOIN
(
SELECT m.senderId, m.receiverId, MAX(m.time) AS MyMaxTime FROM messages GROUP BY m.senderId, m.receiverId
) myLittleQuery
ON messages.senderid = myLittleQuery.senderid AND messages.senderid = myLittleQuery.receiverId AND messages.time = myLittleQuery.MyMaxTime
LEFT JOIN messages_recipients AS recipients ON messages.id = recipients.messageid
LEFT JOIN users AS senders ON messages.senderid = senders.id
LEFT JOIN users AS receivers ON recipients.userid = receivers.id
WHERE recipients.userid = $userid
OR messages.senderid = $userid
GROUP BY messages.id
ORDER BY messages.time DESC
Been a while since I've been in the MySQL realm so my syntax is most likely off. However, the gist of it is there.
Also, depending on what you're trying to retrieve, you might be able to ditch the DISTINCT and CASE statements as well.