SQL 中的 LEFT JOIN 计数

Question

我想知道如何在 SQL 中获得 LEFT JOIN 的计数。

目前我有3张桌子:

rooms_messages

id	房间_id	讨论_消息_id	创建于
1	10	101	2024-07-16 12:30:45
2	20	102	2024-07-16 12:30:50
3	10	103	2024-07-16 12:32:45
4	20	104	2024-07-16 12:34:50
5	20	105	2024-07-16 12:36:50

discussions_messages

（有很多

discussions_replies

）

id	文字
101	你好
102	测试
103	好的
104	哇
105	你好2

discussions_replies

id	讨论_消息_id	文字
201	103	你好
202	103	测试

我使用此查询来显示最新的 rooms_messages 并附加 discussions_messages 文本。但回复不正确：

WITH cte AS (
    SELECT rm.id as id, rm.created_at, rm.room_id,
           rm.discussion_message_id, dm.text, count(dm.id) as replies,
    ROW_NUMBER() OVER (PARTITION BY rm.room_id ORDER BY rm.created_at DESC) rn
    FROM rooms_messages AS rm
    INNER JOIN discussions_messages AS dm ON rm.discussion_message_id = dm.id
    LEFT JOIN discussions_replies AS dr ON dm.id = dr.discussion_message_id
    GROUP BY dm.id
)

SELECT id, created_at, room_id, discussion_message_id, text, replies
FROM cte
WHERE rn = 1

我想显示附加在 discussions_messages 上的 discussions_replies。

我觉得问题出在线上：

count(dm.id) as replies,
...
INNER JOIN discussions_messages AS dm ON rm.discussion_message_id = dm.id
LEFT JOIN discussions_replies AS dr ON dm.id = dr.discussion_message_id

Answer 1

使用第二个 CTE 来获取计数，而不是在获取最后一行的同一 CTE 中执行此操作。

WITH cte AS (
    SELECT rm.id as id, rm.created_at, rm.room_id,
           rm.discussion_message_id, dm.text,
    ROW_NUMBER() OVER (PARTITION BY rm.room_id ORDER BY rm.created_at DESC) rn
    FROM rooms_messages AS rm
    INNER JOIN discussions_messages AS dm ON rm.discussion_message_id = dm.id
    LEFT JOIN discussions_replies AS dr ON dm.id = dr.discussion_message_id
), counts AS (
  SELECT room_id, IFNULL(COUNT(dr.id), 0) AS replies
  FROM rooms_messages AS rm
  INNER JOIN discussions_messages AS dm ON rm.discussion_message_id = dm.id
  LEFT JOIN discussions_replies AS dr ON dm.id = dr.discussion_message_id
  GROUP BY room_id
)

SELECT id, created_at, cte.room_id, discussion_message_id, text, replies
FROM cte 
JOIN counts ON cte.room_id = counts.room_id
WHERE rn = 1

演示