mysql三表join联查

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select *,(select title from messages where id=a.message_id) as message_title,(select name from users where user_id=a.message_user_id) as user_name from comments as a where a.message_user_id = xxx;

select message_user_id, messages.title, users.name from comments, messages, users  while     message_id = messages.id and user_id = users.id     and message_user_id = $id