計算最後一個關聯記錄具有負面屬性的位置

Question

我有兩個表，conversation和message（對話有很多消息）。

conversation: 
    - id 

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

我想算對話，其中最後一個（由message.created_at訂購）消息的read屬性是false。

Answer 1

只得到計數爲概述：

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

你確實不需要包括父表conversation這裏都沒有。

子查詢返回每個對話的最後一行（"ordered by messages.created_at"）。
詳情在這裏：Select first row in each GROUP BY group?
外部查詢計數那些read是FALSE。

替代與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 
    ); 

可能是大表更快。你必須用EXPLAIN ANALYZE進行測試。