Relations, with primary keys underlined:

Users(username, nickname, contact, signupDate)
Hosts(username)
Channels(cname, description, creator, createTime)
Subscribes(username, cname, subscribeTime)
Messages(messID, messageBody, poster, channel, postTime, replyToMessID)
SubscribeHistory(username, subscribeTime, cname, unsubscribeTime)

Where there are the following foreign keys:

• In relation Hosts, username references Users;
• In relation Channels, creator references Hosts;
• In relation Subscribes, username references Users, cname references Channels;
• In relation Messages, (poster, channel) references Subscribes, replyToMessID references messages;
• In relation SubscribeHistory, username references Users, cname references Channels.