SQL檢索最新記錄，按唯一外鍵分組

Question

我創建查詢以使用SQL DB檢索論壇中的最新帖子。

我有一張名爲「Post」的表格。每篇文章與「線程」和「用戶」以及創建日期都有外鍵關係。

訣竅是我不想在同一個線程中顯示同一用戶或兩個帖子的兩篇文章。是否有可能創建一個包含所有這些邏輯的查詢？

# Grab the last 10 posts. 
SELECT id, user_id, thread_id 
FROM posts 
ORDER BY created_at DESC 
LIMIT 10; 

# Grab the last 10 posts, max one post per user 
SELECT id, user_id, thread_id 
FROM post 
GROUP BY user_id 
ORDER BY date DESC 
LIMIT 10; 

# Grab the last 10 posts, max one post per user, max one post per thread??? 

Answer 1

試試這個，看看是否有幫助：

SELECT DISTINCT 
id, user_id, thread_id 
FROM posts 
ORDER BY created_at DESC LIMIT 10; 

SELECT DISTINCT 
id, user_id, thread_id 
FROM post 
GROUP BY user_id 
ORDER BY date DESC 
LIMIT 10; 

您還可以看到一個tutorial關於這一點，一個discussion這一點。

乾杯！ :)

Answer 2

我沒有測試過這一點，但是這給一試：

(
    SELECT p1.id, p1.user_id, p1.thread_id 
    FROM post AS p1 LEFT OUTER JOIN post AS p2 
    ON (p1.user_id = p2.user_id AND p1.date < p2.date) 
    WHERE p2.id IS NULL 
    ORDER BY p1.date DESC 
    LIMIT 10 
) 
UNION DISTINCT 
(
    SELECT p3.id, p3.user_id, p3.thread_id 
    FROM post AS p3 LEFT OUTER JOIN post AS p4 
    ON (p3.thread_id = p4.thread_id AND p3.date < p4.date) 
    WHERE p4.id IS NULL 
    ORDER BY p3.date DESC 
    LIMIT 10 
) 
ORDER BY date DESC 
LIMIT 10; 

Answer 3

這個是什麼？每個用戶的第一個查詢，每個用戶和每個線程的第二個查詢。

SELECT id, user_id, thread_id 
FROM post p1 
WHERE id = (SELECT id 
      FROM post 
      WHERE user_id = p1.user_id 
      ORDER BY date DESC 
      LIMIT 1) 
ORDER BY date DESC 
LIMIT 10; 

SELECT id, user_id, thread_id 
FROM post p1 
WHERE id = (SELECT id 
      FROM post 
      WHERE user_id = p1.user_id 
      ORDER BY date DESC 
      LIMIT 1) 
AND id = (SELECT id 
      FROM post 
      WHERE thread_id = p1.thread_id 
      ORDER BY date DESC 
      LIMIT 1) 
ORDER BY date DESC 
LIMIT 10;