SQL Server 2012

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CREATE TABLE TableA
    ([ID] int, [RequestNumber] int, [Date] datetime)
INSERT INTO TableA
    ([ID], [RequestNumber], [Date])
VALUES
    (1, 1, '2017-09-27 09:30:00'),
    (2, 1, '2018-06-02 09:30:00')
2 rows affected

CREATE TABLE TableB
    ([RequestNumber] int, [Serial] int, [Date] datetime)
INSERT INTO TableB
    ([RequestNumber], [Serial], [Date])
VALUES
    (1, 1, '2017-09-27 09:30:00'),
    (1, 2, '2017-09-27 09:30:00'),
    (1, 6, '2018-06-03 09:30:00'),
    (1, 7, '2018-06-03 09:30:00'),
    (1, 8, '2018-06-03 09:30:00')
5 rows affected

SELECT a.ID, MIN(ABS(DATEDIFF(day, b.Date, a.Date))) AS days
FROM TableA a
JOIN TableB b ON b.RequestNumber = a.RequestNumber
GROUP BY a.ID, a.RequestNumber


ID
days


1
0

2
1


WITH cte (ID, days) AS 
(SELECT a.ID AS ID, MIN(ABS(DATEDIFF(day, b.Date, a.Date))) AS days
 FROM TableA a
 JOIN TableB b ON b.RequestNumber = a.RequestNumber
 GROUP BY a.ID)
SELECT a.ID, a.RequestNumber, b.Serial, b.Date 
FROM TableA a
JOIN TableB b ON b.RequestNumber = a.RequestNumber
JOIN cte ON cte.ID = a.ID AND cte.days = ABS(DATEDIFF(day, b.Date, a.Date))


ID
RequestNumber
Serial
Date


1
1
1
27/09/2017 09:30:00

1
1
2
27/09/2017 09:30:00

2
1
6
03/06/2018 09:30:00

2
1
7
03/06/2018 09:30:00

2
1
8
03/06/2018 09:30:00


SELECT a.ID, a.RequestNumber, b.Serial, b.Date 
FROM TableA a
JOIN TableB b ON b.RequestNumber = a.RequestNumber
JOIN (SELECT a.ID AS ID, MIN(ABS(DATEDIFF(day, b.Date, a.Date))) AS days
      FROM TableA a
      JOIN TableB b ON b.RequestNumber = a.RequestNumber
      GROUP BY a.ID) c  ON c.ID = a.ID AND c.days = ABS(DATEDIFF(day, b.Date, a.Date))


ID
RequestNumber
Serial
Date


1
1
1
27/09/2017 09:30:00

1
1
2
27/09/2017 09:30:00

2
1
6
03/06/2018 09:30:00

2
1
7
03/06/2018 09:30:00

2
1
8
03/06/2018 09:30:00

ID	RequestNumber	Serial	Date
1	1	1	27/09/2017 09:30:00
1	1	2	27/09/2017 09:30:00
2	1	6	03/06/2018 09:30:00
2	1	7	03/06/2018 09:30:00
2	1	8	03/06/2018 09:30:00