How to generate a dynamic sequence in Oracle - sql

Question

I have a table A which represents a valid sequence of numbers, which looks something like this:
| id | start | end | step |
|----|-------|-------|------|
| 1 | 4000 | 4999 | 4 |
| 2 | 3 | 20000 | 1 |
A[1] thus represents the sequence [4000, 4004, 4008, ...4996]
and another B of "occupied" numbers that looks like this:
| id | number | ... |
|-----|--------|-----|
| 1 | 4000 | ... |
| 2 | 4003 | ... |
| ... | ... | ... |
I want to construct a query which using A and B, finds the first unoccupied number for a particular sequence.
I have been trying – and failing – to do, is to generate a list of valid numbers from a row in A and then left outer join table B on B.number = valid_number where B.id is null from which result I could then select min(...).

Answer 1

How about this?
I simplified your test case (END value isn't that high) in order to save space (otherwise, I'd have to use smaller font :)).
What does it do?
CTEs A and B are your sample data
FULL_ASEQ creates a sequence of numbers from table A
if you want what it returns, remove everything from line #17 and - instead of it - run select * from full_aseq
the final query returns the first available sequence number, i.e. the one that hasn't been used yet (lines #19 - 23).
Here you go:
SQL> with
2 a (id, cstart, cend, step) as
3 (select 1, 4000, 4032, 4 from dual union all
4 select 2, 3, 20, 1 from dual
5 ),
6 b (id, cnumber) as
7 (select 1, 4000 from dual union all
8 select 1, 4004 from dual union all
9 select 2, 4003 from dual
10 ),
11 full_aseq as
12 (select a.id, a.cstart + column_value * a.step seq_val
13 from a cross join table(cast(multiset(select level from dual
14 connect by level <= (a.cend - a.cstart) / a.step
15 ) as sys.odcinumberlist))
16 )
17 select f.id, min(f.seq_val) min_seq_val
18 from full_aseq f
19 where not exists (select null
20 from b
21 where b.id = f.id
22 and b.cnumber = f.seq_val
23 )
24 group by f.id;
ID MIN_SEQ_VAL
---------- -----------
1 4008
2 4
SQL>

Answer 2

You can use LEAD to compute the difference between ordered rows in table B. Any row having a difference (to the next row) that exceeds the step value for that sequence is a gap.
Here's that concept, implemented (below). I threw in a sequence ID "3" that has no values in table B, to illustrate that it generates the proper first value.
with
a (id, cstart, cend, step) as
(select 1, 4000, 4032, 4 from dual union all
select 2, 3, 20000, 1 from dual union all
select 3, 100, 200, 3 from dual
),
b (id, cnumber) as
(select 1, 4000 from dual union all
select 1, 4004 from dual union all
select 1, 4012 from dual union all
select 2, 4003 from dual
),
work1 as (
select a.id,
b.cnumber cnumber,
lead(b.cnumber,1) over ( partition by b.id order by b.cnumber ) - b.cnumber diff,
a.step,
a.cstart,
a.cend
from a left join b on b.id = a.id )
select w1.id,
CASE WHEN min(w1.cnumber) is null THEN w1.cstart
WHEN min(w1.cnumber)+w1.step < w1.cend THEN min(w1.cnumber)+w1.step
ELSE null END next_cnumber
from work1 w1
where ( diff is null or diff > w1.step )
group by w1.id, w1.step, w1.cstart, w1.cend
order by w1.id
+----+--------------+
| ID | NEXT_CNUMBER |
+----+--------------+
| 1 | 4008 |
| 2 | 4004 |
| 3 | 100 |
+----+--------------+
You can further improve the results by excluding rows in table B that are impossible for the sequence. E.g., exclude a row for ID #1 having a value of, say, 4007.

Answer 3

I'll ask the obvious and suggest why not use an actual sequence?
SQL> set timing on
SQL> CREATE SEQUENCE SEQ_TEST_A
START WITH 4000
INCREMENT BY 4
MINVALUE 4000
MAXVALUE 4999
NOCACHE
NOCYCLE
ORDER
Sequence created.
Elapsed: 00:00:01.09
SQL> CREATE SEQUENCE SEQ_TEST_B
START WITH 3
INCREMENT BY 1
MINVALUE 3
MAXVALUE 20000
NOCACHE
NOCYCLE
ORDER
Sequence created.
Elapsed: 00:00:00.07
SQL> -- get nexvals from A
SQL> select seq_test_a.nextval from dual
NEXTVAL
----------
4000
1 row selected.
Elapsed: 00:00:00.09
SQL> select seq_test_a.nextval from dual
NEXTVAL
----------
4004
1 row selected.
Elapsed: 00:00:00.08
SQL> select seq_test_a.nextval from dual
NEXTVAL
----------
4008
1 row selected.
Elapsed: 00:00:00.08
SQL> -- get nextvals from B
SQL> select seq_test_b.nextval from dual
NEXTVAL
----------
3
1 row selected.
Elapsed: 00:00:00.08
SQL> select seq_test_b.nextval from dual
NEXTVAL
----------
4
1 row selected.
Elapsed: 00:00:00.08
SQL> select seq_test_b.nextval from dual
NEXTVAL
----------
5
1 row selected.
Elapsed: 00:00:00.08