SQL - find row with closest date but different column value - sql

Question

i'm new to SQL and i would need an help.
I have a TAB and I need to find for any item B in the TAB the item A with the closest date. In this case the A with 02.09.2021 04:25:30
Date.
Item
07.09.2021 05:02:05
A
06.09.2021 05:01:02
A
05.09.2021 05:00:02
A
04.09.2021 04:59:01
A
03.09.2021 04:58:03
A
02.09.2021 04:56:55
A
02.09.2021 04:33:56
B
02.09.2021 04:25:30
A

Answer 1

WITH CTE(DATE,ITEM)AS
(
SELECT '20210907 05:02:05' , 'A'UNION ALL
SELECT '20210906 05:01:02' , 'A'UNION ALL
SELECT '20210905 05:00:02' , 'A'UNION ALL
SELECT'20210904 04:59:01' , 'A'UNION ALL
SELECT'20210903 04:58:03' , 'A'UNION ALL
SELECT'20210902 04:56:55' , 'A'UNION ALL
SELECT'20210902 04:33:56' , 'B'UNION ALL
SELECT'20210902 04:25:30' , 'A'
)
SELECT
CAST(C.DATE AS DATETIME)X_DATE,C.ITEM,Q.CLOSEST
FROM CTE AS C
OUTER APPLY
(
SELECT TOP 1 CAST(X.DATE AS DATETIME)CLOSEST
FROM CTE AS X
WHERE X.ITEM='A'AND CAST(X.DATE AS DATETIME)<CAST(C.DATE AS DATETIME)
ORDER BY CAST(X.DATE AS DATETIME) ASC
)Q
WHERE C.ITEM='B'
You can use OUTER APPLY-approach as in the above query.
Please also take a look that datetime-column (DATE)is written in the ISO-compliant form

Answer 2

Your data has only two columns. If you want the only the closest A timestamp, then the fastest way is probably window functions:
select t.*,
(case when prev_a_date is null then next_a_date
when next_a_date is null then prev_a_date
when datediff(second, prev_a_date, date) <= datediff(second, date, next_a_date) then prev_a_date
else next_a_date
end) as a_date
from (select t.*,
max(case when item = 'A' then date end) over (order by date) as prev_a_date,
min(case when item = 'A' then date end) over (order by date desc) as next_a_date
from t
) t
where item = 'B';
This uses seconds to measure the time difference, but you can use a smaller unit if appropriate.
You can also do this using apply if you have more columns from the "A" rows that you want:
select tb.*, ta.*
from t b outer apply
(select top (1) ta.*
from t ta
where item = 'A'
order by abs(datediff(second, a.date, b.date))
) t
where item = 'B';