Question

我有两个表，conversation和message（会话有很多消息）。

conversation:
  - id

message:
  - id
  - conversation_id
  - created_at
  - read (boolean)

我想计算最后一次（按message.created_at排序}消息的read属性为false的对话。

Answer 1

按照概述得到的计数：

SELECT count(*) AS msg_ct
FROM  (
   SELECT DISTINCT ON (conversation_id) *
   FROM   messages
   ORDER  BY conversation_id, created_at DESC
   ) sub
WHERE NOT read;

您根本不需要在此处包含父表conversation。

子查询返回每个对话的最后一行（"ordered by messages.created_at"）详情请点：Select first row in each GROUP BY group?
外部查询计算read为FALSE的位置。

替代NOT EXISTS：

SELECT count(*) AS msg_ct
FROM   messages m
WHERE  NOT read
AND NOT EXISTS (
   SELECT 1
   FROM   messages m1
   WHERE  m1.conversation_id = m.conversation_id
   AND    m1.created_at > m.created_at
   );

大桌子可能会更快。您必须使用EXPLAIN ANALYZE进行测试。

计算最后关联记录具有负属性的位置

1 个答案: