POSTGRESQL：如何選擇每組的第一行？

Question

有了這個查詢：

WITH responsesNew AS
(
  SELECT DISTINCT responses."studentId", notation, responses."givenHeart", 
  SUM(notation + responses."givenHeart") OVER (partition BY responses."studentId" 
  ORDER BY responses."createdAt") AS total, responses."createdAt",  
  FROM responses
)
SELECT responsesNew."studentId", notation, responsesNew."givenHeart", total, 
responsesNew."createdAt"
FROM responsesNew
WHERE total = 3
GROUP BY responsesNew."studentId", notation, responsesNew."givenHeart", total, 
responsesNew."createdAt"
ORDER BY responsesNew."studentId" ASC

我得到這個數據表：

studentId | notation | givenHeart | total |      createdAt     |
----------+----------+------------+-------+--------------------+
 374      | 1        | 0          | 3     | 2017-02-13 12:43:03   
 374      | null     | 0          | 3     | 2017-02-15 22:22:17
 639      | 1        | 2          | 3     | 2017-04-03 17:21:30 
 790      | 1        | 0          | 3     | 2017-02-12 21:12:23
 ...

我的目標是只在我的數據表中保留每個組的前一行，如下所示：

studentId | notation | givenHeart | total |      createdAt     |
----------+----------+------------+-------+--------------------+
 374      | 1        | 0          | 3     | 2017-02-13 12:43:03 
 639      | 1        | 2          | 3     | 2017-04-03 17:21:30 
 790      | 1        | 0          | 3     | 2017-02-12 21:12:23
 ...

我要怎么去那兒？

我在這里閱讀了很多主題，但我沒有嘗試過DISTINCT 、 DISTINCT ON 、 WHERE 、 LIMIT子查詢等對我有用（當然是由於我的理解不足）。 我遇到了與窗口函數相關的錯誤， ORDER BY缺少列以及其他一些我不記得的錯誤。

Answer 1

您可以使用distinct on做到這一點。 查詢將如下所示：

WITH responsesNew AS (
      SELECT DISTINCT r."studentId", notation, r."givenHeart", 
             SUM(notation + r."givenHeart") OVER (partition BY r."studentId" 
                                                  ORDER BY r."createdAt") AS total,
             r."createdAt" 
      FROM responses r
     )
SELECT DISTINCT ON (r."studentId") r."studentId", notation, r."givenHeart", total, 
r."createdAt"
FROM responsesNew r
WHERE total = 3
ORDER BY r."studentId" ASC, r."createdAt";

我很確定這可以簡化。 我只是不明白 CTE 的目的。 以這種方式使用SELECT DISTINCT非常奇怪。

如果您想要一個簡化的查詢，請提出另一個問題，其中包含示例數據、所需結果以及您正在做什么的說明，並包括查詢或此問題的鏈接。

Answer 2

使用 Row_number() 窗口函數為每個分區添加一個行號，然后只顯示第 1 行。

如果只涉及一張表，則無需完全限定名稱。 並在限定時使用別名以簡化可讀性。

WITH responsesNew AS
(
  SELECT "studentId"
       , notation
       , "givenHeart"
       , SUM(notation + "givenHeart") OVER (partition BY "studentId" ORDER BY "createdAt") AS total
       , "createdAt"
       , Row_number() OVER ("studentId" ORDER BY "createdAt") As RNum
  FROM responses r
)
SELECT RN."studentId"
     , notation, RN."givenHeart"
     , total
     , RN."createdAt"
FROM responsesNew RN
WHERE total = 3
  AND RNum = 1
GROUP BY RN."studentId"
       , notation
       , RN."givenHeart", total
       , RN."createdAt"
ORDER BY RN."studentId" ASC