create table t (n number(3));

insert into t values (1);
insert into t values (2);
insert into t values (3);
insert into t values (4);
insert into t values (7);

insert into t values (9);
insert into t values (10);
 

Start   end
1        4
7        7
9        10

SCOTT@orcl> SELECT * FROM t ORDER BY n
  2  /

         N
----------
         1
         2
         3
         4
         7
         9
        10

7 rows selected.

SCOTT@orcl> SELECT s.n "Start", e.n "end"
  2  FROM   (SELECT n,
  3  		    ROW_NUMBER () OVER (ORDER BY n) rn
  4  	     FROM   t t1
  5  	     WHERE  NOT EXISTS
  6  		    (SELECT n
  7  		     FROM   t t2
  8  		     WHERE  t1.n = t2.n + 1)) s,
  9  	    (SELECT n,
 10  		    ROW_NUMBER () OVER (ORDER BY n) rn
 11  	     FROM   t t1
 12  	     WHERE  NOT EXISTS
 13  		    (SELECT n
 14  		     FROM   t t2
 15  		     WHERE  t1.n = t2.n - 1)) e
 16  WHERE  s.rn = e.rn
 17  ORDER  BY s.n
 18  /

     Start        end
---------- ----------
         1          4
         7          7
         9         10

3 rows selected.

with a as (
           select  n - dense_rank() over(order by n) grp,
                   n
             from  t
          )
select  min(n) start_n,
        max(n) end_n
  from  a
  group by grp
  order by grp
/

   START_N      END_N
---------- ----------
         1          4
         7          7
         9         10

SQL>

SQL> WITH DATA AS(
  2  SELECT n,
  3        n-Row_Number() OVER(ORDER BY n)
  4        AS grp
  5        FROM t
  6        )
  7  SELECT MIN(n),MAX(n)
  8  FROM DATA
  9  GROUP BY grp
 10  ORDER BY MIN(n);

    MIN(N)     MAX(N)
---------- ----------
         1          4
         7          7
         9         10

SQL>

SQL> select * from t order by n;
         N
----------
         1
         2
         3
         3
         4
         7
         9
        10

8 rows selected.

SQL> with a as (
  2             select  n - dense_rank() over(order by n) grp,
  3                     n
  4               from  t
  5            )
  6  select  min(n) start_n,
  7          max(n) end_n
  8    from  a
  9    group by grp
 10    order by grp
 11  /
   START_N      END_N
---------- ----------
         1          4
         7          7
         9         10

3 rows selected.

SQL> WITH DATA AS(
  2  SELECT n,
  3        n-Row_Number() OVER(ORDER BY n)
  4        AS grp
  5        FROM t
  6        )
  7  SELECT MIN(n),MAX(n)
  8  FROM DATA
  9  GROUP BY grp
 10  ORDER BY MIN(n);
    MIN(N)     MAX(N)
---------- ----------
         1          3
         3          4
         7          7
         9         10

4 rows selected.