Solve Sudoku in SQL

postgres.sql

WITH RECURSIVE x( s, ind ) AS
( SELECT sud, position( '.' IN sud )
  FROM  (SELECT '91.7......326.9.8...7.8.9...86.3.17.3.......6.51.2.84...9.5.3...2.3.149......2.61'::text 
             AS sud) xx
  UNION ALL
  SELECT substr( s, 1, ind - 1 ) || z || substr( s, ind + 1 )
       , position('.' IN repeat('x',ind) || substr( s, ind + 1 ) )
  FROM x
     ,  (SELECT gs::text AS z FROM generate_series(1,9) gs) z
  WHERE ind > 0
  AND NOT EXISTS ( SELECT NULL
                   FROM generate_series(1,9) lp
                   WHERE z.z = substr( s, ( (ind - 1 ) / 9 ) * 9 + lp, 1 )
                   OR    z.z = substr( s, mod( ind - 1, 9 ) - 8 + lp * 9, 1 )
                   OR    z.z = substr( s, mod( ( ( ind - 1 ) / 3 ), 3 ) * 3
                                      + ( ( ind - 1 ) / 27 ) * 27 + lp
                                      + ( ( lp - 1 ) / 3 ) * 6
                                   , 1 )
                 )
)
SELECT s
FROM x
WHERE ind = 0;

-- solves this sudoku 
-- 91.  7..  ...
-- .32  6.9  .8.
-- ..7  .8.  9..

-- .86  .3.  17.
-- 3..  ...  ..6
-- .51  .2.  84.

-- ..9  .5.  3..
-- .2.  3.1  49.
-- ...  ..2  .61

sqlite.sql

WITH RECURSIVE
  input(sud) AS (
    VALUES('91.7......326.9.8...7.8.9...86.3.17.3.......6.51.2.84...9.5.3...2.3.149......2.61')
  ),
  digits(z, lp) AS (
    VALUES('1', 1)
    UNION ALL SELECT
    CAST(lp+1 AS TEXT), lp+1 FROM digits WHERE lp<9
  ),
  x(s, ind) AS (
    SELECT sud, instr(sud, '.') FROM input
    UNION ALL
    SELECT
      substr(s, 1, ind-1) || z || substr(s, ind+1),
      instr( substr(s, 1, ind-1) || z || substr(s, ind+1), '.' )
     FROM x, digits AS z
    WHERE ind>0
      AND NOT EXISTS (
            SELECT 1
              FROM digits AS lp
             WHERE z.z = substr(s, ((ind-1)/9)*9 + lp, 1)
                OR z.z = substr(s, ((ind-1)%9) + (lp-1)*9 + 1, 1)
                OR z.z = substr(s, (((ind-1)/3) % 3) * 3
                        + ((ind-1)/27) * 27 + lp
                        + ((lp-1) / 3) * 6, 1)
         )
  )
SELECT s FROM x WHERE ind=0;

-- solves this sudoku 
-- 91.  7..  ...
-- .32  6.9  .8.
-- ..7  .8.  9..

-- .86  .3.  17.
-- 3..  ...  ..6
-- .51  .2.  84.

-- ..9  .5.  3..
-- .2.  3.1  49.
-- ...  ..2  .61