Php 加入3个用户表、朋友表、聊天表

我只想获得单个用户的朋友的结果，并让每个朋友加入用户和朋友之间聊天的最后一个时间戳：

用户好友列表：

• Friend1:fname，lname，…，聊天时间戳

• Friend2:fname，lname，…，聊天时间戳

用户

表是主要的用户信息表：

usr_id，usr_fname，usr_lname

friends

表是一个包含友谊的表：

frd_id、frd_usr_id1、frd_usr_id2

chats

表是一个包含用户之间聊天的表：

cht_id、cht_usr_id1、cht_usr_id2、cht_时间戳

有人能帮我处理SQL语句吗？

---

假设请求的用户ID在$user\u ID变量中

试试这个，它应该可以工作

SELECT u1.usr_fname [Friend1First], 
    u1.usr_lname [Friend1Last], 
    u2.usr_fname [Friend2First], 
    u2.usr_lname [Friend2Last], 
    MAX(ch.cht_timestamp) [LastChatTime]
FROM Chats ch
    JOIN users u1 ON ch.cht_usr_id1 = u1.usr_id
    JOIN users u2 ON ch.cht_usr_id2 = u2.usr_id
WHERE EXISTS (SELECT frd_id from friends fs WHERE (fs.frd_usr_id1 = a.usr_id AND fs.frd_usr_id2 = b.usr_id) 
                    OR (fs.frd_usr_id2 = a.usr_id AND fs.frd_usr_id1 = b.usr_id))
GROUP BY u1.usr_fname, u1.usr_lname, u2.usr_fname, u2.usr_lname,

答案是：

SELECT f.frd_id, u.usr_id, u.usr_fname, u.usr_lname, MAX(c.cht_timestamp)
FROM friends AS f
LEFT JOIN users AS u ON u.usr_id IN (
SELECT u1.usr_id FROM users AS u1 WHERE u1.usr_id != $user_id)
LEFT JOIN chats AS c ON ( (c.cht_usr_id1=$user_id AND c.cht_usr_id2=u.usr_id) OR
      (c.cht_usr_id1=u.usr_id AND c.cht_usr_id2=$user_id) )
    WHERE ( (f.frd_usr_id1=$user_id AND f.frd_usr_id2=u.usr_id) OR
    (f.frd_usr_id1=u.usr_id AND f.frd_usr_id2=$user_id) )
    GROUP BY u.usr_id ORDER BY  MAX(c.cht_timestamp) DESC