Recursive With Clause

with x( s, ind ) as
( select sud, instr( sud, ' ' )
  from ( select 
  '53  7    6  195    98    6 8   6   34  8 3  17   2   6 6    28    419  5    8  79'
  sud from dual )
  union all
  select substr( s, 1, ind - 1 ) || z || substr( s, ind + 1 )
       , instr( s, ' ', ind + 1 )
  from x
     , ( select to_char( rownum ) z
         from dual
         connect by rownum <= 9
       ) z
  where ind > 0
  and not exists ( select null
                   from ( select rownum lp
                          from dual
                          connect by rownum <= 9
                        )
                   where z = substr( s, trunc( ( ind - 1 ) / 9 ) * 9 + lp, 1 )
                   or    z = substr( s, mod( ind - 1, 9 ) - 8 + lp * 9, 1 )
                   or    z = substr( s, mod( trunc( ( ind - 1 ) / 3 ), 3 ) * 3
                                      + trunc( ( ind - 1 ) / 27 ) * 27 + lp
                                      + trunc( ( lp - 1 ) / 3 ) * 6
                                   , 1 )
                 )
)
select s
from x
where ind = 0
/

(Anton Scheffer) [Solving a Sudoku using Recursive Subquery Factoring]

Impressive code from our amici over at Amis. But also an impressive example of how far one can go outside the original domain of a language. I mean recursive queries? On the other hand this algorithm does not look too good in Scala or in Perl either. The mathematics of Sudoku are studied in depth. Sudoku has been shown to be NP-complete and of the many ways of solving Sudoku, dancing links and constraint programming seem to be very popular. (Typically, it takes milli seconds for a computer to solve a Sudoku.)