SQL Server 2016

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drop table if exists Orders;

create table Orders
(SellerID    varchar(10)  not NULL
,OrderID     int          not NULL
,OID         int              NULL);

insert into Orders values
('seller1',123456,null),
('seller1',123457,null),
('seller1',123458,123460),
('seller1',123459,123460),
('seller2',234567,null),
('seller1',123455,null),
('seller2',234568,234570),
('seller2',234569,234570),
('seller1',123463,null),
('seller1',123466,null),
('seller1',123453,null),
('seller2',234572,null),
('seller1',123470,null);
13 rows affected

--select * from Orders;
/*
select SellerID,OrderID from Orders
union
select SellerID,OID     from Orders where OID is not NULL
order by 1,2;
*/
with

-- build a list of all OrderIDs

allids (SellerID, OrderID)
as
(select SellerID,
        OrderID
 from   Orders
 union
 select SellerID,
        OID
 from   Orders
 where  OID is not NULL),

-- find the first 'missing' OrderID for each gap

gapStart (SellerID, startOrderID)
as
(select a1.SellerID,
        a1.OrderID + 1
        
 from   allids a1
 where  not exists(select 1
                   from   allids a2
                   where  a2.OrderID = a1.OrderID + 1)),
             
-- now find the end of each gap

gaps (SellerID, startOrderID, endOrderID)
as
(select gs.SellerID,
        gs.startOrderID,
        min(OrderID) - 1
        
 from   gapStart gs
 
 join   allids   a
 on     a.SellerID = gs.SellerID
 and    a.OrderID  > gs.startOrderID
 
 group by gs.SellerID,
          gs.startOrderID)
            
-- display the gaps

select g.SellerID,
       g.startOrderID + n.rn - 1 as OrderID
       
from   gaps g

-- generate a sequence (1..N) where N = difference between gaps.startOrderID and gaps.endOrderID

cross
apply (select top(g.endOrderID - g.startOrderID + 1)
              row_number() over (order by (select NULL)) as rn
       from   Orders) as n

order by 1,2;


SellerID
OrderID


seller1
123454

seller1
123461

seller1
123462

seller1
123464

seller1
123465

seller1
123467

seller1
123468

seller1
123469

seller2
234571

SellerID	OrderID
seller1	123454
seller1	123461
seller1	123462
seller1	123464
seller1	123465
seller1	123467
seller1	123468
seller1	123469
seller2	234571