How to create teble 3 from data of table 1 and table 2

t1 (ordered descending):

a	comment
512	(ordered desc)
415
310
205
195
150
132
90
74
20
1

t2 (ordered ascending):

b	comment
10	(ordered asc)
11
12
13

I would like to create t3:

a (desc)	b	comment
512	10	(b=asc)
415	11
310	12
205	13
195	13	(b=desc)
150	12
132	11
90	10
74	10	(b=asc)
20	11
1	12

etc.

In table t3, a is data from t1, and b is data from t2.

Answer

You can add a sequence number and then join using arithmetic:

select t1.*, t2.*
from (select t1.*,
             row_number() over (order by a desc) as seqnum
      from t1
     ) t1 left join
     (select t2.*,
             row_number() over (order by which,
                                         case when which = 'a' then b end asc,
                                         b desc
                               ) as seqnum,
             count(*) over () as cnt
      from t2 cross join
           (select 'a' as which from dual union all
            select 'd' as which from dual
           ) x
     ) t2
     on mod(t1.seqnum - 1, t2.cnt) = t2.seqnum - 1
order by t1.seqnum;