SYBASE: SQL call to find first available (island) ID's - sql

Question

I need to return an ordered list of specific length of available customerIDs.
for example:
I need to find the FIRST 5 unused customerIDs between 1500 and 3000
Table= customer
Column= customerIDs
customerIDs value's= 1500,1502,1503,1507,1508
Return desired= 1501,1504,1505,1506,1509
I am running an old version of SYBASE that does not have "TOP" command. So far I have found the following Query to give me the next available customerID(1501), but do not know how to modify it to return the first 5 results rather then just 1.
set rowcount 5
SELECT MIN(c.customerIDs )+1 AS NextID
FROM customer c
WHERE NOT EXISTS
(SELECT NULL
FROM customer cu
WHERE cu.customerIDs =c.customerIDs +1
AND cu.customerIDs >1500)
AND c.customerIDs <3000

Answer 1

Well assuming you are using set rowcount 5, this will limit the query to return 5 results.
However, you are using MIN which will for sure return only 1 record. I think you want to use
set rowcount 5
SELECT c.customerIDs +1 AS NextID
FROM customer c
WHERE (c.customerIDs + 1 BETWEEN 1500 and 3000)
AND c.customerIDs + 1 NOT IN (SELECT c2.customerIDs
FROM customer c2)
ORDER BY c.customerIDs

Answer 2

Here's a worked example, expanding on David Faber's comment:
-- Create a temp table with all possible IDs
declare #min int, #max int
select #min = 1500, #max = 3000
create table #all_ids (id int)
insert #all_ids select #min
-- Keep doubling the number of rows until the limit is reached
while (select max(id) from #all_ids) < #max
begin
insert #all_ids
select id + (select count(*) from #all_ids)
from #all_ids
where id + (select count(*) from #all_ids) <= #max
end
-- Now find the 5 missing IDs by joining to the customer table on customerIDs, and only returning rows
-- where there is no match, i.e. customerIDs is null
set rowcount 5
select tmp.id
from #all_ids tmp
left join customer c
on tmp.id = c.customerIDs
where c.customerIDs is null
order by tmp.id
set rowcount 0

Answer 3

One trick to do this is to generate a sequence on the fly from the master..spt_values table, which is usually available in most servers.
This table contains a natural sequence that can be joined to itself to generate an even larger sequence. Example:
create table #customer (customerID int)
go
insert #customer (customerID) values (1500)
insert #customer (customerID) values (1502)
insert #customer (customerID) values (1503)
insert #customer (customerID) values (1507)
insert #customer (customerID) values (1508)
go
set rowcount 5
select customerID
from (
-- Generate a sequence of 1500-3000
select 1500 + p2.number * 1000 + p1.number as customerID
from
master..spt_values p1
, master..spt_values p2
where
p1.type = 'P'
and p2.type = 'P'
and p1.number between 0 and 999
and p2.number between 0 and 1
and p2.number * 1000 + p1.number <= 1500) seq
where customerID not in (select customerID from #customer)
order by 1
set rowcount 0
go
Returns:
customerID
-------------
1501
1504
1505
1506
1509