要仅选择相互关联的用户/消息,您可以使用
SELECT u.UserId, u.FirstName, u.LastName, m.MessageId, m.Heading, m.Body
FROM User_Messages h
INNER JOIN User u ON h.UserId = u.UserId
INNER JOIN Messages m ON h.MessageId = m.MessageId
要选择所有用户(并添加他们的消息,如果有的话),请使用:
SELECT u.UserId, u.FirstName, u.LastName, m.MessageId, m.Heading, m.Body
FROM User u
LEFT JOIN User_Messages u ON h.UserId = u.UserId
LEFT JOIN Messages m ON h.MessageId = m.MessageId
最后,要选择消息并为每个消息添加用户信息,请使用:
SELECT u.UserId, u.FirstName, u.LastName, m.MessageId, m.Heading, m.Body
FROM Messages m
LEFT JOIN User_Messages h ON h.MessageId = m.MessageId
LEFT JOIN User u ON h.UserId = u.UserId