Hibernate query to get existing conversations between 2 users - mysql

Question

I have 2 tables to denote a conversation, I think it is a pretty standard format.
CREATE TABLE conversation (
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(40) DEFAULT '',
last_modified TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY(id)
) ENGINE = InnoDB;
CREATE TABLE conversation_user (
user_id INT NOT NULL,
conversation_id INT NOT NULL,
conversation_is_visible BOOLEAN NOT NULL DEFAULT TRUE,
unread BOOLEAN NOT NULL DEFAULT TRUE,
PRIMARY KEY (user_id, conversation_id),
CONSTRAINT fk_conversation_user_user_id FOREIGN KEY (user_id) REFERENCES user (id),
CONSTRAINT fk_conversation_user_conversation_id FOREIGN KEY (conversation_id) REFERENCES conversation (id)
) ENGINE = InnoDB;
I want to be able to check if a conversation exists between 2 users (I don't care about starting a new conversation for more than 2 users if it already exists).
I can't seem to be able to write a query to select the conversation with just the 2 given users in it, and no more.
I then want to convert the query into a hibernate query.
For example if i had the following data:
select * from conversation;
+----+------------------------+---------------------+
| id | name | last_modified |
+----+------------------------+---------------------+
| 1 | test conversation 1 | 2017-02-25 15:39:01 |
| 2 | test conversation 2 | 2017-02-26 16:29:14 |
+----+------------------------+---------------------+
select * from conversation_user;
+---------+-----------------+-------------------------+--------+
| user_id | conversation_id | conversation_is_visible | unread |
+---------+-----------------+-------------------------+--------+
| 1 | 1 | 1 | 0 |
| 1 | 2 | 1 | 0 |
| 2 | 1 | 1 | 1 |
| 2 | 2 | 1 | 0 |
| 3 | 2 | 1 | 0 |
+---------+-----------------+-------------------------+--------+
The query should when searching for a conversation between user 1 and user 2 only have conversation 1 show up, as conversation 2 has user 3 in it aswell.
I've tried the following query, but that shows up both conversations, so not sure how to restrict it to conversations just containing those users...
SELECT * FROM conversation
LEFT JOIN conversation_user ON conversation.id = conversation_user.conversation_id
WHERE user_id IN (1,2);
I could use this query in hibernate and then check the set of conversation users in the conversation to make sure the conversation only contains the 2 given users, but was wondering if there was a more efficient approach

Answer 1

Try this
http://sqlfiddle.com/#!9/587e4/5
SELECT t1.user_id,
t1.conversation_id,
t1.conversation_is_visible,
t1.unread
FROM conversation_user t1
LEFT OUTER JOIN
(SELECT user_id,conversation_id
FROM conversation_user WHERE user_id IN (1,2)
) t2 ON t1.user_id=t2.user_id
AND t1.conversation_id=t2.conversation_id
WHERE t2.conversation_id IS NOT NULL;
If you just want conversation_id then use distinct t1.conversation_id in select clause.

Answer 2

Based on Utsav's answer, the following query got the results i was after.
SELECT t1.user_id, t1.conversation_id, t1.conversation_is_visible, t1.unread
FROM conversation_user t1
LEFT OUTER JOIN
(SELECT *
FROM conversation_user
GROUP BY conversation_id HAVING COUNT(*) = 2
) t2
ON t1.user_id = t2.user_id AND t1.conversation_id=t2.conversation_id
WHERE t1.user_id IN (1,2) AND t2.conversation_id IS NOT NULL;
still not sure how to convert this to a query in hibernate but its a start