PostgreSQL：如何select每个帐户在给定日期范围内每一天的最后余额？

Question

我是运行 PostgreSQL 9.3 并且有一个 table 看起来像这样：

     entry_date      | account_id | balance
---------------------+------------+---------
 2016-02-01 00:00:00 |        123 |     100
 2016-02-01 06:00:00 |        123 |     200
 2016-02-01 12:00:00 |        123 |     300
 2016-02-01 18:00:00 |        123 |     250
 2016-02-01 00:00:00 |        456 |     400
 2016-02-01 06:00:00 |        456 |     300
 2016-02-01 12:00:00 |        456 |     200
 2016-02-01 18:00:00 |        456 |     299
 2016-02-02 00:00:00 |        123 |     250
 2016-02-02 06:00:00 |        123 |     300
 2016-02-02 12:00:00 |        123 |     400
 2016-02-02 18:00:00 |        123 |     450
 2016-02-02 00:00:00 |        456 |     299
 2016-02-02 06:00:00 |        456 |     200
 2016-02-02 12:00:00 |        456 |     100
 2016-02-02 18:00:00 |        456 |       0
(16 rows)

我的目标是在给定日期范围内的每一天检索每个帐户的最终余额。所以我想要的结果是：

     entry_date      | account_id | balance
---------------------+------------+---------
 2016-02-01 18:00:00 |        123 |     250
 2016-02-01 18:00:00 |        456 |     299
 2016-02-02 18:00:00 |        123 |     450
 2016-02-02 18:00:00 |        456 |       0
(4 rows)

请注意，我示例中的时间戳比实际中的时间戳要整齐得多...我不能总是依赖 18:00 作为每天的最后时间。

我该如何编写这个 SQL 查询？

我试过这个的变体：

SELECT max(entry_date), account_id, max(balance)
FROM ledger
WHERE entry_date BETWEEN '2016-02-01'::timestamp AND '2016-02-02'::timestamp
GROUP BY account_id, entry_date;

架构如下：

CREATE TABLE ledger (
  entry_date    timestamp(3),
  account_id    int,
  balance       int
);

INSERT INTO ledger VALUES ('2016-02-01T00:00:00.000Z', 123, 100);
INSERT INTO ledger VALUES ('2016-02-01T06:00:00.000Z', 123, 200);
INSERT INTO ledger VALUES ('2016-02-01T12:00:00.000Z', 123, 300);
INSERT INTO ledger VALUES ('2016-02-01T18:00:00.000Z', 123, 250);

INSERT INTO ledger VALUES ('2016-02-01T00:00:00.000Z', 456, 400);
INSERT INTO ledger VALUES ('2016-02-01T06:00:00.000Z', 456, 300);
INSERT INTO ledger VALUES ('2016-02-01T12:00:00.000Z', 456, 200);
INSERT INTO ledger VALUES ('2016-02-01T18:00:00.000Z', 456, 299);

INSERT INTO ledger VALUES ('2016-02-02T00:00:00.000Z', 123, 250);
INSERT INTO ledger VALUES ('2016-02-02T06:00:00.000Z', 123, 300);
INSERT INTO ledger VALUES ('2016-02-02T12:00:00.000Z', 123, 400);
INSERT INTO ledger VALUES ('2016-02-02T18:00:00.000Z', 123, 450);

INSERT INTO ledger VALUES ('2016-02-02T00:00:00.000Z', 456, 299);
INSERT INTO ledger VALUES ('2016-02-02T06:00:00.000Z', 456, 200);
INSERT INTO ledger VALUES ('2016-02-02T12:00:00.000Z', 456, 100);
INSERT INTO ledger VALUES ('2016-02-02T18:00:00.000Z', 456, 0);

这是一个SQLFiddle：http://sqlfiddle.com/#!15/56886

提前致谢！

Answer 1

在 Postgres 中，我认为最简单的方法是 distinct on:

SELECT DISTINCT ON (account_id) l.*
FROM ledger l
WHERE entry_date BETWEEN '2016-02-01'::timestamp AND '2016-02-02'::timestamp
ORDER BY account_id, entry_date DESC;

DISTINCT ON 根据 ORDER BY 中的键对数据进行排序。然后它选择 ON 列表中键的唯一值，选择遇到的第一个值。

编辑：

完全同样的想法适用于一天的一条记录——我只是误读了原始要求：

SELECT DISTINCT ON (account_id, date_trunc('day', entry_date)) l.*
FROM ledger l
WHERE entry_date BETWEEN '2016-02-01'::timestamp AND '2016-02-02'::timestamp
ORDER BY account_id, date_trunc('day', entry_date), entry_date DESC;

Answer 2

您可以将 ROW_NUMBER 与 PARTITION BY 一起使用：

SELECT entry_date, account_id, balance
FROM (
  SELECT entry_date, account_id, balance, 
         ROW_NUMBER() OVER (PARTITION BY account_id, entry_date::date 
                            ORDER BY entry_date DESC) AS rn
  FROM ledger
  WHERE entry_date BETWEEN '2016-02-01'::timestamp AND '2016-02-02'::timestamp) AS t
WHERE t.rn = 1

PARTITION BY 每天创建 account_id 个值的切片，因为 entry_date 也用于同一子句 在转换为日期值后 .每个切片按 entry_date 降序排列，因此 ROW_NUMBER = 1 对应于当天的最后一条记录。

Demo here

PostgreSQL：如何select每个帐户在给定日期范围内每一天的最后余额？

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