如何将一个表中的最新行连接到另一个表中？

StackOverflow https://stackoverflow.com/questions/497535

题

我的数据看起来像这样：

entities
id         name
1          Apple
2          Orange
3          Banana

流程将定期运行并为每个实体提供分数。该过程生成数据并将其添加到分数表中，如下所示：

scores 
id  entity_id    score   date_added
1    1            10       1/2/09
2    2            10       1/2/09
3    1            15       1/3/09
4    2            10       1/03/09
5    1            15       1/4/09
6    2            15       1/4/09
7    3            22       1/4/09

我希望能够选择所有实体以及每个实体的最新记录分数，从而产生如下数据：

entities
id name     score  date_added
1  Apple     15     1/4/09
2  Orange    15     1/4/09
3  Banana    15     1/4/09

我可以使用此查询获取单个实体的数据：

SELECT entities.*, 
       scores.score, 
       scores.date_added 
FROM entities

INNER  JOIN scores
ON entities.id = scores.entity_id

WHERE entities.id = ?

ORDER BY scores.date_added DESC
LIMIT 1

但我不知道如何为所有实体选择相同的内容。也许它正盯着我的脸？

非常感谢您抽出时间。

感谢您的精彩回复。我将给它几天时间，看看是否出现首选解决方案，然后我将选择答案。

更新：我已经尝试了几种建议的解决方案，我现在面临的主要问题是，如果实体尚未生成分数，它们就不会出现在列表中。

SQL 会是什么样子来确保返回所有实体，即使它们还没有发布任何分数？

更新：已选择答案。感谢大家！

有帮助吗？

解决方案

我做这种方式：

SELECT e.*, s1.score, s1.date_added 
FROM entities e
  INNER JOIN scores s1
    ON (e.id = s1.entity_id)
  LEFT OUTER JOIN scores s2
    ON (e.id = s2.entity_id AND s1.id < s2.id)
WHERE s2.id IS NULL;

其他提示

我想补充我的变化就可以了：

SELECT e.*, s1.score
FROM entities e
INNER JOIN score s1 ON e.id = s1.entity_id
WHERE NOT EXISTS (
    SELECT 1 FROM score s2 WHERE s2.id > s1.id
)

<强>方法1 

SELECT entities.*, 
       scores.score, 
       scores.date_added 
FROM entities

INNER  JOIN scores
ON entities.id = scores.entity_id

WHERE scores.date_added = 
  (SELECT max(date_added) FROM scores where entity_id = entities.id)

方法2

相对于批次的查询成本：

SELECT entities.*, 
       scores.score, 
       scores.date_added 
FROM entities

INNER  JOIN scores
ON entities.id = scores.entity_id

inner join 
    (
    SELECT 
           entity_id, max(date_added) as recent_date
    FROM scores
    group by entity_id
    ) as y on entities.id = y.entity_id and scores.date_added = y.recent_date

我知道这是一个老问题，只是想我要补充没有人提到的是，Cross Apply或Outer Apply一个方法。这是在SQL Server 2005（数据库类型不标记在这个问题上）或更高

可用

使用临时表

DECLARE @Entities TABLE(Id INT PRIMARY KEY, name NVARCHAR(MAX))
INSERT INTO @Entities
VALUES (1, 'Apple'), (2, 'Orange'), (3, 'Banana'), (4, 'Cherry')

DECLARE @Scores TABLE(Id INT PRIMARY KEY, Entity_Id INT, Score INT, Date_Added DATE)
INSERT INTO @Scores
VALUES (1,1,10,'2009-02-01'),
(2,2,10,'2009-02-01'),
(3,1,15,'2009-02-01'),
(4,2,10,'2009-03-01'),
(5,1,15,'2009-04-01'),
(6,2,15,'2009-04-01'),
(7,3,22,'2009-04-01')

您可以使用

SELECT E.Id, E.name, S.Score, S.Date_Added 
FROM @Entities E
CROSS APPLY
(
    SELECT TOP 1 * 
    FROM @Scores Sc 
    WHERE Sc.Entity_Id = E.Id  
    ORDER BY sc.Score DESC
) AS S

，以获得所需的结果。所述equivilent以允许无得分的实体将是

SELECT E.Id, E.name, S.Score, S.Date_Added 
FROM @Entities E
OUTER APPLY
(
    SELECT TOP 1 * 
    FROM @Scores Sc 
    WHERE Sc.Entity_Id = E.Id  
    ORDER BY sc.Score DESC
) AS S

SELECT entities.*, 
       scores.score, 
       scores.date_added 
FROM entities

INNER  JOIN scores
ON entities.id = scores.entity_id

WHERE entities.id in 
(select id from scores s2 where date_added = max(date_added) and s2.id = entities.id)

ORDER BY scores.date_added DESC
LIMIT 1

您也可以使用窗口功能，如ROW_NUMBER做到这一点今天在大多数的RDBMS（甲骨文和PostgreSQL，SQL Server）的具有天然查询：

SELECT id, name, score, date_added FROM (
 SELECT e.id, e.name, s.score, s.date_added,
 ROW_NUMBER() OVER (PARTITION BY e.id ORDER BY s.date_added DESC) rn
 FROM Entities e INNER JOIN Scores s ON e.id = s.entity_id
) tmp WHERE rn = 1;

的 SQL小提琴

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