T-SQL Get percentage of character match of 2 strings

Question

Let\'s say I have a set of 2 words:

Alexander and Alecsander OR Alexander and Alegzander

Alexander and Aleaxnder, or any other combination. In general we a

Answer 1

Ultimately, you appear to be looking to solve for the likelihood that two strings are a "fuzzy" match to one another.

SQL provides efficient, optimized built-in functions that will do that for you, and likely with better performance than what you have written. The two functions you are looking for are SOUNDEX and DIFFERENCE.

While neither of them solves exactly what you asked for - i.e. they do not return a percentage match - I believe they solve what you are ultimately trying to achieve.

SOUNDEX returns a 4-character code which is the first letter of the word plus a 3-number code that represents the sound pattern of the word. Consider the following:

SELECT SOUNDEX('Alexander')
SELECT SOUNDEX('Alegzander')
SELECT SOUNDEX('Owleksanndurr')
SELECT SOUNDEX('Ulikkksonnnderrr')
SELECT SOUNDEX('Jones')

/* Results:

A425
A425
O425
U425
J520

*/

What you will notice is that the three-digit number 425 is the same for all of the ones that roughly sound alike. So you could easily match them up and say "You typed 'Owleksanndurr', did you perhaps mean 'Alexander'?"

In addition, there's the DIFFERENCE function, which compares the SOUNDEX discrepancy between two strings and gives it a score.

SELECT DIFFERENCE(  'Alexander','Alexsander')
SELECT DIFFERENCE(  'Alexander','Owleksanndurr')
SELECT DIFFERENCE(  'Alexander', 'Jones')
SELECT DIFFERENCE(  'Alexander','ekdfgaskfalsdfkljasdfl;jl;asdj;a')

/* Results:

4
3
1
1     

*/

As you can see, the lower the score (between 0 and 4), the more likely the strings are a match.

The advantage of SOUNDEX over DIFFERENCE is that if you really need to do frequent fuzzy matching, you can store and index the SOUNDEX data in a separate (indexable) column, whereas DIFFERENCE can only calculate the SOUNDEX at the time of comparison.

Answer 2

Ok, here is my solution so far:

SELECT  [dbo].[GetPercentageOfTwoStringMatching]('valentin123456'  ,'valnetin123456')

returns 86%

CREATE FUNCTION [dbo].[GetPercentageOfTwoStringMatching]
(
    @string1 NVARCHAR(100)
    ,@string2 NVARCHAR(100)
)
RETURNS INT
AS
BEGIN

    DECLARE @levenShteinNumber INT

    DECLARE @string1Length INT = LEN(@string1)
    , @string2Length INT = LEN(@string2)
    DECLARE @maxLengthNumber INT = CASE WHEN @string1Length > @string2Length THEN @string1Length ELSE @string2Length END

    SELECT @levenShteinNumber = [dbo].[LEVENSHTEIN] (   @string1  ,@string2)

    DECLARE @percentageOfBadCharacters INT = @levenShteinNumber * 100 / @maxLengthNumber

    DECLARE @percentageOfGoodCharacters INT = 100 - @percentageOfBadCharacters

    -- Return the result of the function
    RETURN @percentageOfGoodCharacters

END




-- =============================================     
-- Create date: 2011.12.14
-- Description: http://blog.sendreallybigfiles.com/2009/06/improved-t-sql-levenshtein-distance.html
-- =============================================

CREATE FUNCTION [dbo].[LEVENSHTEIN](@left  VARCHAR(100),
                                    @right VARCHAR(100))
returns INT
AS
  BEGIN
      DECLARE @difference    INT,
              @lenRight      INT,
              @lenLeft       INT,
              @leftIndex     INT,
              @rightIndex    INT,
              @left_char     CHAR(1),
              @right_char    CHAR(1),
              @compareLength INT

      SET @lenLeft = LEN(@left)
      SET @lenRight = LEN(@right)
      SET @difference = 0

      IF @lenLeft = 0
        BEGIN
            SET @difference = @lenRight

            GOTO done
        END

      IF @lenRight = 0
        BEGIN
            SET @difference = @lenLeft

            GOTO done
        END

      GOTO comparison

      COMPARISON:

      IF ( @lenLeft >= @lenRight )
        SET @compareLength = @lenLeft
      ELSE
        SET @compareLength = @lenRight

      SET @rightIndex = 1
      SET @leftIndex = 1

      WHILE @leftIndex <= @compareLength
        BEGIN
            SET @left_char = substring(@left, @leftIndex, 1)
            SET @right_char = substring(@right, @rightIndex, 1)

            IF @left_char <> @right_char
              BEGIN -- Would an insertion make them re-align?
                  IF( @left_char = substring(@right, @rightIndex + 1, 1) )
                    SET @rightIndex = @rightIndex + 1
                  -- Would an deletion make them re-align?
                  ELSE IF( substring(@left, @leftIndex + 1, 1) = @right_char )
                    SET @leftIndex = @leftIndex + 1

                  SET @difference = @difference + 1
              END

            SET @leftIndex = @leftIndex + 1
            SET @rightIndex = @rightIndex + 1
        END

      GOTO done

      DONE:

      RETURN @difference
  END