How to find range in oracle

Question

I am trying to find the range in following problem

I have a table named my_tble(id varchar(100),roll_no varchar(100)) with following values.

-------------------
|id    | roll_no  |
-------------------
| 001  |     1    |
| 002  |     2    |
| 003  |     3    |
| 004  |     4    |
| 005  |     7    |
| 006  |     8    |
| 007  |     11   |
| 008  |     15   |
| 009  |     16   |
| 010  |     17   |
-------------------

Output should be in this way:

-----------
|roll_no  |
--------- 
| 1-4     |
| 7-8     |
| 11-11   |
| 15-17   |
-----------

Answer 1

with base as (select 001 id, 1 roll_no from dual
                            union all
                            select  002 id, 2 roll_no from dual
                            union all
                            select  003 id, 3 roll_no from dual
                            union all
                            select  004 id, 4 roll_no from dual
                            union all
                            select  005 id, 7 roll_no from dual
                            union all
                            select  006 id, 8 roll_no from dual
                            union all
                            select  007 id, 11 roll_no from dual
                            union all
                            select  008 id, 15 roll_no from dual
                            union all
                            select  009 id, 16 roll_no from dual
                            union all
                            select  010 id, 17    roll_no from dual)
select min(roll_no)||' - '|| max(roll_no) roll_no
from (select roll_no
            ,connect_by_root roll_no root
            from base
            connect by prior roll_no = roll_no-1
            start with roll_no not in (select roll_no
                                       from base b 
                                       where exists (select 1
                                       from base b1
                                       where b1.roll_no = b.roll_no-1))) 
group by root
order by root

SQL Fiddle

Also you can find another approach with LAG analytic function