在MySQL中選擇最常出現的值

Question

我正在尋找一種方法來選擇最常出現的值，例如，每個線程發佈最多的人;

SELECT MOST_OCCURRING(user_id) FROM thread_posts GROUP BY thread_id 

有沒有一種很好的方法來做到這一點？

Answer 1

如果你想在每個線程的基礎上計數，我認爲你可以使用嵌套查詢;按線程先分組，然後按用戶分組：

SELECT thread_id AS tid, 
    (SELECT user_id FROM thread_posts 
     WHERE thread_id = tid 
     GROUP BY user_id 
     ORDER BY COUNT(*) DESC 
     LIMIT 0,1) AS topUser 
FROM thread_posts 
GROUP BY thread_id 

Answer 2

如果您在「每個組最多的n個」標籤下檢查問題，有很多示例。但在這種情況下，您並未定義您想如何處理關係 - 如果兩個或更多用戶具有相同的計數值，該怎麼辦？

SELECT DISTINCT 
     tp.thread_id, 
     tp.user_id 
    FROM THREAD_POSTS tp 
    JOIN (SELECT t.thread_id, 
       t.user_id, 
       COUNT(t.user_id) AS occurrence, 
       CASE 
       WHEN @thread != t.thread_id THEN @rownum := 1 
       ELSE @rownum := @rownum + 1 
       END AS rank, 
       @thread := t.thread_id 
      FROM THREAD_POSTS t 
      JOIN (SELECT @rownum := 0, @thread := -1) r 
     GROUP BY t.thread_id, t.user_id 
     ORDER BY t.thread_id, occurrence DESC) x ON x.thread_id = tp.thread_id 
               AND x.user_id = tp.user_id 
               AND x.rank = 1 

Answer 3

這將製表USER_ID的出現每線程

SELECT thread_id, user_id, COUNT(*) as postings 
FROM thread_posts 
GROUP BY thread_id, user_id 

但你只是想選擇頂級用戶爲每個線程

SELECT thread_id, user_id, postings 
FROM (
    SELECT thread_id, user_id, COUNT(*) as postings 
    FROM thread_posts 
    GROUP BY thread_id, user_id 
) 
HAVING postings = max(postings) 

這相當於

SELECT thread_id, user_id, COUNT(*) as postings 
FROM thread_posts 
GROUP BY thread_id, user_id 
HAVING postings = max(postings) 

HAVING keywor d通常與聚合操作一起使用，以挑選滿足HAVING子句中條件的聚合輸出行。

HAVING子句與WHERE子句不同，其中HAVING子句過濾查詢的結果輸出。而WHERE子句會過濾查詢的輸入數據。 由於HAVING子句過濾查詢的結果輸出，因此它必須出現在ORDER BY和GROUP BY子句之後。