找到联系人最多的用户

Question

用户向对方消息框发送消息，如何找到知道最多其他用户的用户（接收和发送??）。我有SQL Server。

表：

• USER_ID
• USER_NAME
• USER_GROUP
• MESSAGE_ID
• MESSAGE_MESSAGE
• MESSAGE_FROM_USER
• M_ESSAGE_TO_USER

用户可以发送和接收，因此谁知道其他大多数用户（通过发送和接收？）

Answer 1

SQL Server 2005+，使用CTE：

---

 WITH usr_list AS (
   SELECT x.user_id,
          x.message_from_user AS other_user
     FROM TABLE x
   UNION
   SELECT y.user_id,
          y.message_to_user AS other_user
     FROM TABLE y)
  SELECT TOP 1
         ul.user_id,
         COUNT(*) AS num_friends
    FROM usr_list ul
GROUP BY ul.user_id
ORDER BY num_friends DESC

非CTE当量：

---

  SELECT TOP 1
         ul.user_id,
         COUNT(*) AS num_friends
    FROM (SELECT x.user_id,
                 x.message_from_user AS other_user
            FROM TABLE x
          UNION
          SELECT y.user_id,
                 y.message_to_user AS other_user
            FROM TABLE y) ul
GROUP BY ul.user_id
ORDER BY num_friends DESC

Answer 2

未经测试，但这应该确保每个其他用户只计算一次，无论他们是发件人还是收件人，或者他们发送或接收了多少邮件：

SELECT TOP 1
  user_id,
  friend_count = COUNT(DISTINCT u)
FROM
(
  SELECT user_id,
    u = message_from_user
    FROM table
  UNION ALL
  SELECT user_id,
    u = message_to_user
    FROM table
) AS x
ORDER BY friend_count DESC;