MySQL查詢訂購

Question

這是我的數據。我想採用6行，但我希望所有HeadlineCategoryId在我的結果列表中都是唯一的。如果我選擇前6名，我會從HeadlineCategoryID 20（6,2）中取2排。你有什麼建議嗎？

Answer 1

SELECT a.* 
FROM tableName a 
     INNER JOIN 
     (
      SELECT HeadlineCategoryID, MAX(Creation) max_date 
      FROM TableName 
      GROUP BY HeadlineCategoryID 
     ) b ON a.HeadlineCategoryID = b.HeadlineCategoryID AND 
       a.Creation = b.max_date 
ORDER BY a.Creation DESC -- << specify here how are you going to sort 
LIMIT 6     -- the records you want to get 

---

更新1

SELECT a.* 
FROM tableName a 
     INNER JOIN 
     (
      SELECT HeadlineCategoryID, MAX(NewsID) max_id 
      FROM TableName 
      GROUP BY HeadlineCategoryID 
     ) b ON a.HeadlineCategoryID = b.HeadlineCategoryID AND 
       a.NewsID = b.max_id 
ORDER BY a.Creation DESC -- << specify here how are you going to sort 
LIMIT 6     -- the records you want to get 

Answer 2

它看起來像你想的六個最近的記錄，但HeadlineCategoryId獨特。如果是這樣，這將工作：

select top 6 NewsId, Creation, HeadlineCategoryId 
from (select t.*, 
      row_number() over (partition by HeadlineCategoryId order by Creation desc) as seqnum 
     from t 
    ) t 
where seqnum = 1 

作爲一個說明。 。 。這個問題最初表明它使用的是SQL Server，而不是MySQL。 MySQL中的解決方案並不那麼簡單。這裏有一個方法與not exists：

select NewsId, Creation, HeadlineCategoryId 
from t 
where not exists (select 1 
        from t t2 
        where t2.HeadlineCategoryId = t.HeadlineCategoryId and 
         t2.id < t.id) 
limit 6 

的not exists部分說：「那裏沒有其他記錄與給定標題類別更大的ID」。