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.