Need to join 3 select queries that refers to same table - sql

Question

I am using SQL Server Express 2014 and I need to pull out the last record for few (3 for now) tags with different IDs from one table.
So far I made it but not at all. I am using
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
and the result is what I need, but not exactly. I need to get the three results in single table like:
TagItemId TagValue
Random.Int1 55
Random.Int2 75
Random.Int3 23`
and not like:
TagItemId TagValue
Random.Int1 55
TagItemId TagValue
Random.Int2 75
TagItemId TagValue
Random.Int3 23`
The reason is that I need to use the data for a chart.
Best regards and thanks!

Answer 1

You could do this using Row_Number
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

There are several ways to accomplish this:
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')
Or:
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)
Or:
;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

Could you not just do a simple 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 )
Probably crashed because each select TOP had an order by, so I "wrapped" the queries into select * so each subquery could retain its own ORDER BY clause