DECLARE @INPUT nvarchar(100) = 'abbccc';

WITH nums AS
(
   SELECT n = 1           -- base case starting at 1

   UNION ALL

   SELECT n = n+1         -- recursive case:  next number = 2, 3, ... len(@input)
   FROM nums
   WHERE n < len(@INPUT)  -- stop and don't run if n equal the len(@input)
)
,perms AS
(
   SELECT                                      -- base case for recursion
      string = CAST(''     AS nvarchar(100)),  -- string we are generating, start as empty string
      remain = CAST(@INPUT AS nvarchar(100)),  -- remaining characters to append to our string
      pos    = n                               -- position of next character in remaining string to append to base case
   FROM nums

   UNION ALL

   -- recursive case where we append 1 character per step
   SELECT
      -- append character at position pos to base string
      string = CAST(string + SUBSTRING(remain,pos,1) AS nvarchar(100)),
      -- remove character we added from remaining characters
      remain = CAST(stuff(remain,pos,1,'') AS nvarchar(100)),
      -- all possible positions of remaining characters
      pos    = nums.n
   FROM perms, nums
   -- only look at possible valid positions
   -- this forces the recursion to terminate when remain is empty
   WHERE pos <= len(remain)
)
SELECT DISTINCT string
FROM perms
WHERE len(string) = len(@INPUT)
ORDER BY string
OPTION (maxrecursion 10000)