Implementing a conversation style private messaging system

Question

I currently have designed a system for simple send message receive message using such simple table. Now I need an extra information that is which message belongs to which conversations. Any ideas tips or guidelines on implementing this kind of system?

CREATE TABLE messages (
    ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    FromID INT NOT NULL,   
    ToID INT NOT NULL,  
    Deleted BOOLEAN DEFAULT FALSE,
    SentDeleted BOOLEAN DEFAULT FALSE,
    Subject varchar(255), 
    Message varchar(255),
    DateTime DATETIME                                       
    ) ENGINE=InnoDB;

Answer 1

just add an int column called parent_message_id and set it to the id of the message you are replying to.

Answer 2

The more common way, though, is to have a thread table and messages table. When a conversation is started, create a thread record and set the thread_id column of the message record to it. Set all replies' thread_id to that thread as well. That way you can SELECT * FROM messages where thread_id = x