Group by和「first」或「min」

Question

我有一個表「消息」與列id，from，to，datetime，subject，body。我想爲每個發件人（來自）獲取最新消息的主題和正文。

這裏是我的查詢

SELECT min(subject) as tsubject,min(body) as tbody 
FROM messages 
WHERE "to"=28 
GROUP BY "from" 
ORDER BY "datetime" 

這將導致

ERROR: column "messages.datetime" must appear in the GROUP BY clause or be used in an aggregate function 

，所以我得到的錯誤是什麼一回事，但我不希望按「日期時間」的結果，只是要排序的羣體，所以min會從每個組中挑選最新的消息。

Answer 1

SELECT a.*,u.name,m.subject,m.body FROM 
(SELECT "from",max("datetime") as sent 
FROM messages 
WHERE "to" = 31 
GROUP BY "from") a 
LEFT JOIN users u ON a."from" = u.id 
LEFT JOIN messages m ON a.sent=m."datetime" AND a."from"=m."from" 

Answer 2

select subject, body, m."from" 
from (
    SELECT 
     id, 
     "from", 
     max("datetime") 
    FROM messages 
    WHERE "to" = 28 
    group by id, "from" 
    ) s 
    inner join 
    messages m on m.id = s.id 
ORDER BY "datetime" desc