Question

I have two tables which get updated at almost the exact same time – I need to join on the datetime column.

I’ve tried this:

SELECT *
FROM A, B
WHERE ABS(DATEDIFF(second, A.Date_Time, B.Date_Time)) = (
    SELECT MIN(ABS(DATEDIFF(second, A.Date_Time, B2.Date_Time)))
    FROM B AS B2
)

But it tells me:

Multiple columns are specified in an aggregated expression containing an outer reference. If an expression being aggregated contains an outer reference, then that outer reference must be the only column referenced in the expression.

How can I join these tables?

Answer 1

How about something like (assuming SQL 2005+) :

With RankedItems As
    (
    Select A.DateTime As ADateTime, B.DateTime As BDateTime
        , ROW_NUMBER() OVER ( PARTITION BY A.DateTime ORDER BY ABS( DateDiff(s, A.DateTime, B.DateTime) ) ) As ItemRank
    From A
        Cross Join B
    )
Select 
From RankedItems
Where ItemRank = 1

In my solution, I’m using a common-table expression or CTE for short. In the CTE, I’m using a ranking function (ROW_NUMBER) which calculates for each row in the CTE expression. The ROW_NUMBER function will return provide a sequential integer for each DateTime value in table A via the PARTITION BY A.DateTime clause ordered by the absolute value of the “closeness” A.DateTime value to the B.DateTime value. Thus, I’m ranking the “closeness” using Abs(DateDiff(s,A.DateTime, B.DateTime) and choosing the highest rank (rank = 1, aka “closest” value) for each A.DateTime value. It will not matter if there is a tie as the ROW_NUMBER() function will return a unique list of numbers for each A.DateTime value.