PostgreSQL的：一個訂單後返回不同的行BY

Question

我的數據庫：

SELECT * FROM odds ORDER BY created_at DESC; 

runner_id | value |   created_at   
-----------+-------+---------------------------- 
    365910 |  5 | 2017-03-13 14:29:45.404927 
    365911 | 2.9 | 2017-03-13 14:29:45.404927 
    365912 |  7 | 2017-03-13 14:29:45.404927 
    365910 |  5 | 2017-03-13 14:29:02.654532 
    365911 |  3 | 2017-03-13 14:29:02.654532 
    365912 |  7 | 2017-03-13 14:29:02.654532 
    365910 | 5.5 | 2017-03-13 14:28:22.733787 
    365911 |  3 | 2017-03-13 14:28:22.733787 
    365912 |  7 | 2017-03-13 14:28:22.733787 
    365910 | 5.5 | 2017-03-13 14:25:42.885042 
    365911 |  3 | 2017-03-13 14:25:42.885042 
    365912 | 7.5 | 2017-03-13 14:25:42.885042 
    365913 | 10 | 2017-03-13 14:25:42.885042 
    365910 |  5 | 2017-03-13 14:25:21.842275 
    365911 |  3 | 2017-03-13 14:25:21.842275 
    365912 | 7.5 | 2017-03-13 14:25:21.842275 
    365913 | 10 | 2017-03-13 14:25:21.842275 

我要找的DISTINCT runner_id行，具有最高created_at。這是我想要的結果：

runner_id | value |   created_at   
-----------+-------+---------------------------- 
    365910 |  5 | 2017-03-13 14:29:45.404927 
    365911 | 2.9 | 2017-03-13 14:29:45.404927 
    365912 |  7 | 2017-03-13 14:29:45.404927 
    365913 | 10 | 2017-03-13 14:25:42.885042 

這似乎很容易，但我無法把握它。我開始認爲這是不可能的，因爲ORDER BY是最後發生的事情，我需要SQL來對行進行排序，然後在runner_id上選擇DISTINCT。

Answer 1

您的查詢是使用Postgres的分析功能ROW_NUMBER()的理想選擇。您希望保留每個runner_id組記錄中的最新記錄。下面的CTE計算每個邏輯的行數，查詢只保留最新的記錄。

WITH cte AS (
    SELECT runner_id, value, created_at, 
      ROW_NUMBER() OVER (PARTITION BY runner_id ORDER BY created_at DESC) rn 
    FROM odds 
) 
SELECT 
    t.runner_id, 
    t.value, 
    t.created_at 
FROM cte t 
WHERE t.rn = 1 

我們也可以嘗試使用DISTINCT ON這裏：

SELECT DISTINCT ON (runner_id) 
    runner_id, value, created_at 
FROM odds 
ORDER BY runner_id, created_at DESC