Question

我坚持创建SQL查询。我有以下两个表：

member
(
 memberid    int,
 membername  varchar(50)
)

message
(
 messageid   int, 
 message     varchar(100), 
 sender      int, 
 receiver    int
)

sender和receiver是包含表格member和列memberid的外键。我想显示name和id sender和receiver的消息列表。

有人可以帮助我，因为我无法在这里建立适当的联接吗？

Answer 1

您可以尝试类似

的内容

SELECT  *
FROM    message m INNER JOIN
        member snd ON   m.sender = snd.memberid INNER JOIN
        member rcv ON   m.receiver = rcv.memberid

Answer 2

我会在这种情况下使用LEFT OUTER JOIN，因为您的message.sender和message.receiver允许NULL值。如果这些列中的任何一列是NULL：

，您可能不希望跳过消息

select msg.messageid as messageid,
       msg.message as message,
       mem1.memberid as sender_id,
       mem1.membername as sender_name,
       mem2.memberid as receiver_id,
       mem2.membername as receiver_name
from message as msg
left outer join member as mem1 on msg.sender = mem1.memberid
left outer join member as mem2 on msg.receiver = mem2.memberid;

http://sqlfiddle.com/#!3/ac80e/2/0

无法创建正确的SQL

2 个答案: