MySQL / PHP - Conversation System - php

Question

I'm having a really hard time figuring this one out. I am trying to make a conversation system and it would work like this:
A logged in user would have a link to "conversations". On that page a list of all conversation.titles would show up. When a conversation-name is clicked the conversation messages (mostly between 2 users, but could be more) will show up like this:
Conversation tiltle...
|------------------------------------------------------------|
|Send-button| Textfield for new message here |
|----------------|-------------------------------------------|
Sender: Hi....... (08/24/11 - 12PM)
receiver: Hi!..... (08/24/11 - 11PM
Sender: bla blah... (08/24/11 - 10PM)
etc..
I have no problem listing messages in the conversation itself, but rather getting the list of conversations a user participates in.
I get multiple results of the same instance.
I would like a query that produces a list of all conversations a logged in user has participated in either by sending a message, recieving a message or both.
I guess i would need a nested query, but i dont know how to do that. So i'm a bit stuck here, trying to inner-join all tables doesnt cut the cheese.
This is the database mysql design / ER-modell:
-- -----------------------------------------------------
-- Table `firm`.`conversation`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `firm`.`conversation` (
`conversation_id` INT NOT NULL AUTO_INCREMENT ,
`title` VARCHAR(45) NULL ,
`date` TIMESTAMP NULL ,
PRIMARY KEY (`subject_id`) )
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `firm`.`message`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `firm`.`message` (
`message_id` INT NOT NULL AUTO_INCREMENT ,
`message` LONGTEXT NULL ,
`date` TIMESTAMP NOT NULL ,
`read_sender` BINARY NOT NULL ,
`read_receiver` BINARY NOT NULL ,
`deleted_receiver` BINARY NOT NULL ,
`deleted_sender` BINARY NOT NULL ,
`conversation_id` INT NOT NULL ,
PRIMARY KEY (`message_id`) ,
INDEX `fk_message_subject1` (`subject_id` ASC) ,
CONSTRAINT `fk_message_con1`
FOREIGN KEY (`conversation_id` )
REFERENCES `firm`.`conversation` (`conversation_id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
DEFAULT CHARACTER SET = latin1
COLLATE = latin1_swedish_ci;
-- -----------------------------------------------------
-- Table `firm`.`outbox`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `firm`.`outbox` (
`outbox_id` INT NOT NULL AUTO_INCREMENT ,
`user_id` INT(10) NOT NULL ,
`conversation_id` INT(10) NOT NULL ,
`date` TIMESTAMP NOT NULL ,
PRIMARY KEY (`outbox_id`) ,
INDEX `fk_outbox_users1` (`user_id` ASC) ,
INDEX `fk_utbox_con1` (`conversation_id` ASC) ,
CONSTRAINT `fk_outbox_users1`
FOREIGN KEY (`user_id` )
REFERENCES `firm`.`users` (`user_id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_outbox_con1`
FOREIGN KEY (`conversation_id` )
REFERENCES `firm`.`conversation` (`conversation_id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
-- -----------------------------------------------------
-- Table `firm`.`inbox`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `firm`.`inbox` (
`inbox_id` INT NOT NULL ,
`user_id` INT(10) NOT NULL ,
`message_id` INT NOT NULL ,
`read` BINARY NOT NULL ,
`date` TIMESTAMP NOT NULL ,
PRIMARY KEY (`inbox_id`) ,
INDEX `fk_inbox_users1` (`user_id` ASC) ,
INDEX `fk_inbox_message` (`message_id` ASC) ,
CONSTRAINT `fk_inbox_users1`
FOREIGN KEY (`user_id` )
REFERENCES `firm`.`users` (`user_id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_inbox_message1`
FOREIGN KEY (`message_id` )
REFERENCES `frim`.`message` (`message_id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;

Answer 1

SELECT DISTINCT conversation.conversation_id
FROM conversation
INNER JOIN message ON message.conversation_id = conversation.conversation_id
INNER JOIN inbox ON inbox.message_id = message.message_id
INNER JOIN outbox ON inbox.message_id = message.message_id
WHERE outbox.user_id = #USER#
OR inbox.user_id = #USER#
This join should work. DISTINCT because a user is likely to have participated as a sender and a recipient.