with data as(
  select 1 id
  union all
  select 2
  union all
  select 3
  union all
  select 5
)
,seq as(
  select row_number()over(order by id) rn, id from data
)
select s1.id s1_id, s2.id s2_id, s1.rn s1_rn, s2.rn s2_rn
,case when s1.rn=s2.rn-1 then 'yes'  when s1.rn<>s2.rn-1 then 'no!' else 'hm!..' end result
from seq s1
left join seq s2 on s1.rn=s2.rn-1

with data as(
  select 1 id from dual 
  union all
  select 2 from dual  
  union all
  select 3 from dual  
  union all
  select 5 from dual  
)
,seq as(
  select row_number()over(order by id) rn, id from data
)
select s1.id s1_id, s2.id s2_id, s1.rn s1_rn, s2.rn s2_rn
,case when s1.rn=s2.rn-1 then 'yes'  when s1.rn<>s2.rn-1 then 'no!' else 'hm!..' end result
from seq s1
left join seq s2 on s1.rn=s2.rn-1;

     S1_ID      S2_ID      S1_RN      S2_RN RESUL
---------- ---------- ---------- ---------- -----
         1          2          1          2 yes
         2          3          2          3 yes
         3          5          3          4 yes
         5       null          4       null hm!..

SQL> with data as(
CON>   select 1 id from rdb$database union all
CON>   select 2 id from rdb$database union all
CON>   select 3 id from rdb$database union all
CON>   select 5 id from rdb$database
CON> )
CON> ,seq as(
CON>   select row_number()over(order by id) rn, id from data
CON> )
CON> select
CON>     s1.id s1_id, s2.id s2_id
CON>     ,s1.rn s1_rn, s2.rn s2_rn
CON>     ,iif(s1.rn=s2.rn-1,'yes',iif(s1.rn<>s2.rn-1, 'no!', 'hm!..')) result
CON> from seq s1
CON> left join seq s2 on s1.rn=s2.rn-1;

       S1_ID        S2_ID                 S1_RN                 S2_RN RESULT
============ ============ ===================== ===================== ======
           1            2                     1                     2 yes
           2            3                     2                     3 yes
           3            5                     3                     4 yes
           5             5                      4                      0  hm!..