Question

我正在使用SQL Server Express 2014，我需要从一个表中提取少数（现在为3个）不同ID的最后一个记录。

到目前为止，我做到了，但根本没有。我正在使用

SELECT TOP 1 [TagItemId], [TagValue]
FROM [DB].[dbo].[Table] 
where [TagItemId] like 'Random.Int1'
order by [TagTimestamp] desc

SELECT TOP 1 [TagItemId], [TagValue]
FROM [DB].[dbo].[Table] 
where [TagItemId] like 'Random.Int2'
order by [TagTimestamp] desc

SELECT TOP 1 [TagItemId], [TagValue]
FROM [DB].[dbo].[Table] 
where [TagItemId] like 'Random.Int3'
order by [TagTimestamp] desc

结果是我需要的，但不完全是。我需要在单个表中得到三个结果，如：

TagItemId TagValue
Random.Int1     55
Random.Int2     75
Random.Int3     23`

而不喜欢：

TagItemId    TagValue
Random.Int1     55
TagItemId    TagValue
Random.Int2     75
TagItemId    TagValue
Random.Int3     23`

原因是我需要将数据用于图表。

致以最诚挚的问候和感谢！

Answer 1

您可以使用Row_Number

执行此操作

SELECT  [TagItemId],
        [TagValue]
FROM
    (
        SELECT  [TagItemId],
                [TagValue],
                ROW_NUMBER() OVER (PARTITION BY [TagItemId] ORDER BY [TagTimestamp] DESC) Rn
        FROM    [DB].[dbo].[Table]
        WHERE   [TagItemId] IN ('Random.Int1','Random.Int2','Random.Int3')
    ) t
WHERE Rn = 1

Answer 2

有几种方法可以实现这一目标：

SELECT
    MT.TagItemID,
    MT.TagValue
FROM
    My_Table MT
INNER JOIN
(
    SELECT TagItemID, MAX(TagTimestamp)
    FROM My_Table
    WHERE
        MT.TagItemID IN ('Random.Int1', 'Random.Int2', 'Random.Int3')
    GROUP BY TagItemID) SQ ON SQ.TagItemID = MT.TagItemID
WHERE
    MT.TagItemID IN ('Random.Int1', 'Random.Int2', 'Random.Int3')

或者：

SELECT
    MT.TagItemID,
    MT.TagValue
FROM
    My_Table MT
WHERE
    MT.TagItemID IN ('Random.Int1', 'Random.Int2', 'Random.Int3') AND
    NOT EXISTS (SELECT * FROM My_Table MT2 WHERE MT2.TagItemID = MT.TagItemID AND MT2.Timestamp > MT.Timestamp)

或者：

;WITH CTE_WithRowNums AS
(
    SELECT
        MT.TagItemID,
        MT.TagValue,
        ROW_NUMBER() OVER(PARTITION BY TagItemID ORDER BY Timestamp DESC) AS row_num
    FROM
        My_Table MT
)
SELECT
    TagItemID,
    TagValue
FROM
    CTE_WithRowNums
WHERE
    row_num = 1

Answer 3

你能不能只做一个简单的UNION ALL

select * from 
   (SELECT TOP 1 [TagItemId], [TagValue]
       FROM [DB].[dbo].[Table] 
       where [TagItemId] like 'Random.Int1'
       order by [TagTimestamp] desc )
UNION ALL
select * from 
   (SELECT TOP 1 [TagItemId], [TagValue]
       FROM [DB].[dbo].[Table] 
       where [TagItemId] like 'Random.Int2'
       order by [TagTimestamp] desc )
UNION ALL
select * from
    (SELECT TOP 1 [TagItemId], [TagValue]
       FROM [DB].[dbo].[Table] 
       where [TagItemId] like 'Random.Int3'
       order by [TagTimestamp] desc )

可能崩溃了，因为每个选择TOP都有一个order by，所以我将查询“包装”到select *中，这样每个子查询都可以保留自己的ORDER BY子句

需要加入引用同一个表的3个选择查询

3 个答案: