SQL Server Inner Join with Timestamps: is each record only assigned once? - sql

Question

I am working with timestamped records and need to do an inner join based on the timestamp difference. I have been using the DATEDIFF function and it seems to be working well. However, the amount of time between timestamps varies. To clarify, sometimes the record appears in table 2 within the same second as table 1, and sometimes the record in table 2 is up to 15 seconds behind the record in table 1. The records in table 1 are always timestamped before table 2. There is no other common field with which I can join, however there is a register number in each table that I am using to increase accuracy by ensuring that the registers are the same.
My question is: if I increase the timestamp difference to do the inner join (e.g. where the DATEDIFF = 1 or 2 or 3... or 15) will records only be joined once? Or would my table contain duplicate records from table 1 (e.g. where record 1 is joined to record 4 in table 2 where the diff is 4 seconds, and is also joined with record 7 from table 2 where the diff is 11 seconds)?
The reason my statement works now is that no registers have records with less than 6 seconds in between, so even if there are multiple timestamps that would match, the matching of registers eliminates this problem.
My Statement is currently working as:
SELECT *
INTO AtriumSequoiaJoin5
FROM Atrium INNER JOIN Sequoia ON Atrium.Reader = Sequoia.theader_pos_name
WHERE (
((DateDiff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=0
Or (DateDiff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=1
Or (DateDiff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=2
Or (DateDiff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=3
Or (DateDiff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=4
Or (Datediff(s,[Atrium].[Date2],[Sequoia].[theader_tdatetime]))=5)
)
ORDER BY Sequoia.theader_id;

Answer 1

you could CROSS APPLY to the closest record in proximity. That's by no means ideal however, what if there are multiple records written at the same time? You perhaps should give the first table an identity field, then update the next table with scopeidentity
SELECT *
INTO AtriumSequoiaJoin5
FROM Atrium CROSS APPLY
(SELECT TOP 1 * FROM Sequoia WHERE
Atrium.Reader = Sequoia.theader_pos_name
ORDER BY Datediff(millisecond,[Atrium].[Date2],[Sequoia].[theader_tdatetime])) DQ
ORDER BY Sequoia.theader_id;