SQL Server – Generating PERMUTATIONS using T-Sql

Were you ever asked to generate string Permutations using TSql? I was recently asked to do so, and the logic which I could manage to come up at that point is shared in the below script.

DECLARE @Value AS VARCHAR(20) = 'ABCC' --Mention the text which is to be permuted
DECLARE @NoOfChars AS INT = LEN(@Value)
DECLARE @Permutations TABLE (Value VARCHAR(20)) --Make sure the size of this Value is equal to your input  string length (@Value)
 ;

WITH NumTally
AS (
 --Prepare the Tally Table to separate each character of the Value.
 SELECT 1 Num
 
 UNION ALL
 
 SELECT Num + 1
 FROM NumTally
 WHERE Num < @NoOfChars
 ),
Chars
AS (
 --Separate the Characters
 SELECT Num,
  SUBSTRING(@Value, Num, 1) Chr
 FROM NumTally
 )
--Persist the Separated characters.
INSERT INTO @Permutations
SELECT Chr
FROM Chars

--Prepare Permutations
DECLARE @i AS INT = 1

WHILE (@i < @NoOfChars)
BEGIN
 --Store the Permutations
 INSERT INTO @Permutations
 SELECT DISTINCT --Add DISTINCT if required else duplicate Permutations will be generated for Repeated  Chars.
  P1.Value + P2.Value
 FROM (
  SELECT Value
  FROM @Permutations
  WHERE LEN(Value) = @i
  ) P1
 CROSS JOIN (
  SELECT Value
  FROM @Permutations
  WHERE LEN(Value) = 1
  ) P2

 --Increment the Counter.      
 SET @i += 1

 --Delete the Incorrect Lengthed Permutations to keep the table size under control.
 DELETE
 FROM @Permutations
 WHERE LEN(Value) NOT IN (
   1,
   @i
   )
END

--Delete InCorrect Permutations.
SET @i = 1

WHILE (@i <= @NoOfChars)
BEGIN
 --Deleting Permutations which has not used "All the Chars of the given Value".
 DELETE
 FROM @Permutations
 WHERE Value NOT LIKE '%' + SUBSTRING(@Value, @i, 1) + '%'

 --Deleting Permutations which have repeated incorrect character.  
 DELETE
 FROM @Permutations
 WHERE LEN(Value) - LEN(REPLACE(Value, SUBSTRING(@Value, @i, 1), '')) != LEN(@Value) - LEN(REPLACE(@Value, SUBSTRING(@Value, @i, 1), ''))

 SET @i += 1
END

--Selecting the generated Permutations. 
SELECT Value
FROM @Permutations

Hope, this script helps!


Please share your suggestions if you have any to improve this logic.