DECLARE @T TABLE ([ID] int, [Дата поставки] date);

INSERT @T
VALUES (245, CONVERT(date, '12.05.2015', 104))
      ,(245, CONVERT(date, '19.06.2015', 104))
      ,(245, CONVERT(date, '02.07.2015', 104))
      ,(245, CONVERT(date, '15.07.2015', 104))
      ,(345, CONVERT(date, '12.04.2016', 104))
      ,(345, CONVERT(date, '19.06.2016', 104))
      ,(345, CONVERT(date, '03.07.2016', 104))
      ,(345, CONVERT(date, '15.08.2016', 104));

;WITH [CTE]
AS
(
SELECT [ID]
      ,DATEDIFF(day, LAG([Дата поставки]) OVER (PARTITION BY [ID] ORDER BY [Дата поставки]), [Дата поставки]) AS [Diff]
  FROM @T
)
SELECT [ID]
      ,MIN([Diff]) [MinDiff]
  FROM [CTE]
 GROUP BY [ID];