如何将一个表中的最近一行join到另一个表中？
我有这样的数据：
entities id name 1 Apple 2 Orange 3 Banana 定期地，一个过程将运行并给每个实体一个分数。 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select所有的实体以及最近logging的分数，从而得到如下的一些数据：
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
但是我不知道如何为所有实体select相同的东西。 也许它正在盯着我呢？
非常感谢您花时间。
谢谢你的回应。 我会给它几天，看看是否有一个首选的解决scheme泡沫，然后我会select答案。
更新：我尝试了几个build议的解决scheme，我现在面临的主要问题是，如果一个实体还没有生成的分数，他们不会出现在列表中。
SQL看起来像什么来确保所有实体都返回，即使他们还没有发布任何分数？
更新：select答案。 感谢大家！
我这样做：
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;
只是为了添加我的变化：
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 )
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
查询与批次相关的成本：
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方法1 ：22％ – 更快
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方法2：22％ – 更快
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方法3 ：27％
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方法4 ：27％
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，只是认为我会添加一个没有人提到的方法，即Cross Apply或Outer Apply 。 这些在SQL Server 2005中可用（数据库types在此问题中未被标记）或更高
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')
你可以使用
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
得到期望的结果。 equivilent允许没有分数的实体将是
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
您还可以在大多数RDBMS（Oracle，PostgreSQL，SQL Server）中使用窗口函数（如ROW_NUMBER）进行自然查询：
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;
SQL小提琴
