迭代VS.(Iteration Vs. Enumeration)

 我只是将你的联接的where子句移动到ON语句并进行了正确的连接，如果这不起作用我将设置一个sqlfiddle，看看这个sql的问题是什么 
 
SELECT c.*, count(dm.id), 
    u1.first_name, u1.last_name, u1.company, u1.picture, 
    u2.first_name, u2.last_name, u2.company, u2.picture
    FROM "DirectConversation" c
    JOIN "DirectMessage" dm ON c.id = dm."id_DirectConversation" 
    JOIN "Profile" u1 ON u1."id_User" = c."id_User1"
    JOIN "Profile" u2 ON u2."id_User" = c."id_User2"
    WHERE 
    dm.viewed = 'f' AND dm.deleted = 'f'
    AND (c."id_User1" = $id OR c."id_User2" = $id)
    GROUP BY c.id, u1.id, u2.id;
 
 编辑：为了安全起见，将OR子句分组 

I just moved your where clauses for the joins to the ON statement and made proper joins, if this doesn't work I'll set up a sqlfiddle and see what the problem with this sql is
 
SELECT c.*, count(dm.id), 
    u1.first_name, u1.last_name, u1.company, u1.picture, 
    u2.first_name, u2.last_name, u2.company, u2.picture
    FROM "DirectConversation" c
    JOIN "DirectMessage" dm ON c.id = dm."id_DirectConversation" 
    JOIN "Profile" u1 ON u1."id_User" = c."id_User1"
    JOIN "Profile" u2 ON u2."id_User" = c."id_User2"
    WHERE 
    dm.viewed = 'f' AND dm.deleted = 'f'
    AND (c."id_User1" = $id OR c."id_User2" = $id)
    GROUP BY c.id, u1.id, u2.id;
 
Edit: grouped the OR clause just to be safe