Messages: Query a user's chats

问题

For an app like iOS Messages:

1. How should I design the server database queries & schema?

   Queries

   • Create a user.
   • Get a user's chats.
   • Get the messages between two users.
   • Add a message to a chat (creating the chat if it doesn't yet exist).

   Note:

   • Every chat only has two users (no group chats).
   • Every chat's id equals this unique unordered pairing function applied to user_id & with_id.

2. How should I query a user's chats, each with its latest message?

   Example User Interface (UI)

   

See my design & query on SQLFiddle. It works, but how can I make it better?

来源：https://stackoverflow.com/questions/22367688/messages-query-a-users-chats