Oracle: enumerate groups of similar rows - sql

Question

I have the following table:
ID | X
1 | 1
2 | 2
3 | 5
4 | 6
5 | 7
6 | 9
I need to enumerate groups of rows in such way that if row i and i-1 differ in column X by less than 2 they should have the same group number N. See example below.
ID | X | N
1 | 1 | 1
2 | 2 | 1
3 | 5 | 2
4 | 6 | 2
5 | 7 | 2
6 | 9 | 3
Note that rows X(2)-X(1)=1 so they are grouped in the first group. Than X(3)-X(2)=3 so the 3rd row goes to 2nd group with 3rd and 4th row. X(6)-X(5)=2 so 6th row is in the 3rd group.
Can anybody help me with writing SQL query that will return the second table?

Answer 1

This should do it:
select id, x, sum(new_group) over (order by id) as group_no
from
( select id, x, case when x-prev_x = 1 then 0 else 1 end new_group
from
( select id, x, lag(x) over (order by id) prev_x
from mytable
)
);
I get the correct answer for your data with that query.

Answer 2

SQL> create table mytable (id,x)
2 as
3 select 1, 1 from dual union all
4 select 2, 2 from dual union all
5 select 3, 5 from dual union all
6 select 4, 6 from dual union all
7 select 5, 7 from dual union all
8 select 6, 9 from dual
9 /
Table created.
SQL> select id
2 , x
3 , sum(y) over (order by id) n
4 from ( select id
5 , x
6 , case x - lag(x) over (order by id)
7 when 1 then 0
8 else 1
9 end y
10 from mytable
11 )
12 order by id
13 /
ID X N
---------- ---------- ----------
1 1 1
2 2 1
3 5 2
4 6 2
5 7 2
6 9 3
6 rows selected.
Which is essentially the same as Tony's answer, only one inline view less.
Regards,
Rob.

Answer 3

Using basic operations only:
create table test(id int, x int);
insert into test values(1, 1), (2, 2), (3, 5), (4, 6), (5, 7), (6, 9);
create table temp as
select rownum() r, 0 min, x max
from test t
where not exists(select * from test t2 where t2.x = t.x + 1);
update temp t set min = select max + 1 from temp t2 where t2.r = t.r - 1;
update temp t set min = 0 where min is null;
select * from temp order by r;
select t.id, t.x, x.r from test t, temp x where t.x between x.min and x.max;
drop table test;
drop table temp;