一种可能的方法是以下声明：
table：

CREATE TABLE Table1 (Product varchar(1), Item int)
INSERT INTO Table1 (Product, Item)
VALUES ('A', 1), ('A', 2), ('B', 1), ('B', 2), ('B', 3)

CREATE TABLE Table2 (Product varchar(1), Item int)
INSERT INTO Table2 (Product, Item)
VALUES ('A', 1), ('A', 2), ('B', 1)

声明：

SELECT 
   Product, 
   Item, 
   ROW_NUMBER() OVER (PARTITION BY Product ORDER BY Item) AS Rn
FROM Table1

SELECT 
  t2.Product, 
  t2.Item, 
  ROW_NUMBER() OVER (PARTITION BY t2.Product ORDER BY t2.Item) + t1.[Count] AS Rn
FROM Table2 t2
LEFT JOIN (
   SELECT Product, COUNT(*) AS [Count]
   FROM Table1 
   GROUP BY Product
) t1 ON t2.Product = t1.Product

结果：

Product Item  Rn
A       1     1
A       2     2
B       1     1
B       2     2
B       3     3

Product Item Rn
A       1    3
A       2    4
B       1    4