不同字段中最大日期的 SQL Server 函数

Question

我在数据库中有多个相关表，每个表都可以单独更新，并有自己的 LastUpdated Date 字段。在这些表之一中，有多个 LastUpdated 字段，每个字段都指示该记录是从哪个源更新的。我们通常将这些多个表作为带有连接的单个项目进行查询，因此我想知道每条记录的最新 LastUpdated 记录在所有连接中是什么。我知道这可以通过许多子查询来实现，但我想知道是否有一些类似于 Coalesce 函数的东西可以传递许多字段并返回第一个非空值。

所以它会读成这样：

SELECT a.a_id, 
       a.name, 
       b.b_id, 
       b.detail, 
       c.c_id, 
       c.otherfield, 
       Maxdate(a.lastupdated, a.lastupdatedfromweb, b.lastupdated, 
       c.lastupdated) AS 
       LastUpdatedDate 
FROM   a 
       INNER JOIN b 
               ON a.a_id = b.a_id 
       INNER JOIN c 
               ON b.b_id = c.b_id 

有什么想法吗？这可以写成自定义函数还是存在于盒子中？如果有帮助，我正在开发 SQL Server 2005 和 2008。

Answer 1

在 2008+ 年，您可以使用 CROSS APPLY 为您的每个日期创建一行，然后选择 Max：

SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 ld.LastUpdatedDate
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id
   CROSS APPLY
   (    SELECT  LastUpdatedDate = MAX(LastUpdatedDate)
        FROM    (VALUES
                    (a.LastUpdated), 
                    (a.LastUpdatedFromWeb), 
                    (b.LastUpdated), 
                    (c.LastUpdated)
                ) d (LastUpdatedDate)
    ) ld

您也可以将其作为关联子查询执行，但在优化期间 SQL Server 会将关联子查询重写为 OUTER APPLY，因此我更愿意只是跳出一步并自己编写 APPLY，因为我在 SQL 时发现了一些不寻常的行为解构相关子查询并将其重写为 APPLY。这方面的更多细节是described in this answer

对于 2005 年，我认为您需要使用稍微不同的方法，因为它不支持表值构造函数：

SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 ld.LastUpdatedDate,
 (  SELECT  MAX(CASE Number 
                    WHEN 1 THEN a.LastUpdated
                    WHEN 2 THEN a.LastUpdatedFromWeb
                    WHEN 3 THEN b.LastUpdated
                    WHEN 4 THEN c.LastUpdated
                END)
    FROM    (SELECT TOP 4 Number = ROW_NUMBER() OVER(ORDER BY object_id) 
            FROM sys.all_objects) n
) AS LastUpdated
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id;

或者：

SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 ld.LastUpdatedDate,
 (  SELECT  MAX(LastUpdated)
    FROM    (   SELECT a.LastUpdated UNION ALL
                SELECT a.LastUpdatedFromWeb UNION ALL
                SELECT b.LastUpdated UNION ALL
                SELECT c.LastUpdated
            ) d
) AS LastUpdated
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id;

很遗憾，我不再安装任何 2005 实例，因此我无法对此进行测试。

编辑

刚刚意识到你也想要字段的来源，还记得2005确实支持使用APPLY，所以对于2008+：

SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 ld.LastUpdatedDate,
 ld.FieldName
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id
   CROSS APPLY
   (    SELECT  TOP 1 LastUpdatedDate, FieldName
        FROM    (VALUES
                    (a.LastUpdated, 'a.LastUpdated'), 
                    (a.LastUpdatedFromWeb, 'a.LastUpdatedFromWeb'), 
                    (b.LastUpdated, 'b.LastUpdated'), 
                    (c.LastUpdated, 'c.LastUpdated')
                ) d (LastUpdatedDate, FieldName)
        ORDER BY LastUpdated DESC
    ) ld

2005 年：

SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 ld.LastUpdatedDate,
 ld.FieldName
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id
   CROSS APPLY
   (    SELECT  TOP 1 LastUpdatedDate = LastUpdated, FieldName
        FROM    (
                    SELECT a.LastUpdated, FieldName = 'a.LastUpdated' UNION ALL
                    SELECT a.LastUpdatedFromWeb, 'a.LastUpdatedFromWeb' UNION ALL
                    SELECT b.LastUpdated, 'b.LastUpdated' UNION ALL
                    SELECT c.LastUpdated, 'c.LastUpdated'
                ) d
        ORDER BY LastUpdated DESC
    ) ld

Answer 2

 SELECT 
 a.a_Id,
 a.Name,
 b.b_Id,
 b.Detail,
 c.c_Id,
 c.OtherField,
 MAX(SELECT a.LastUpdated, a.LastUpdatedFromWeb, b.LastUpdated, c.LastUpdated FROM a INNER JOIN b ON a.a_Id = b.a_Id INNER JOIN c ON b.b_Id = c.b_Id) AS LastUpdatedDate
FROM 
 a INNER JOIN b ON a.a_Id = b.a_Id 
   INNER JOIN c ON b.b_Id = c.b_Id