Leven-einstein

When having to compare 2 text strings (FI 2 addresses) and get a similarity score, Mr Levenshtein invented an algorithm that is very useful.

The bad news is that  MYSQL do not implement it.
Bytheway you may decide to implement it through a user define function (UDF)... but under windows it would mean a lot of compiling, storing and so on... (in reality I could not succeed but if you want to try see here at Sherlock software)

In LINUX it seems to be much easier (for ubuntu (in german) see http://www.teamarbyte.de/levenshtein.html)

So in my WIN environment I implemented, thanks to CODEJANITOR (see http://codejanitor.com/wp/2007/02/10/levenshtein-distance-as-a-mysql-stored-function/) this plain SQL function... much slower than a compiled one but that is life...

CREATE FUNCTION LEVENSHTEIN (s1 VARCHAR(255), s2 VARCHAR(255))
  RETURNS INT
    DETERMINISTIC
      BEGIN
        DECLARE s1_len, s2_len, i, j, c, c_temp, cost INT;
        DECLARE s1_char CHAR;
        DECLARE cv0, cv1 VARBINARY(256);
        SET s1_len = CHAR_LENGTH(s1), s2_len = CHAR_LENGTH(s2), cv1 = 0x00, j = 1, i = 1, c = 0;
        IF s1 = s2 THEN
          RETURN 0;
        ELSEIF s1_len = 0 THEN
          RETURN s2_len;
        ELSEIF s2_len = 0 THEN
          RETURN s1_len;
        ELSE
          WHILE j <= s2_len DO
            SET cv1 = CONCAT(cv1, UNHEX(HEX(j))), j = j + 1;
          END WHILE;
          WHILE i <= s1_len DO
            SET s1_char = SUBSTRING(s1, i, 1), c = i, cv0 = UNHEX(HEX(i)), j = 1;
            WHILE j <= s2_len DO
                SET c = c + 1;
                IF s1_char = SUBSTRING(s2, j, 1) THEN SET cost = 0; ELSE SET cost = 1; END IF;
                SET c_temp = CONV(HEX(SUBSTRING(cv1, j, 1)), 16, 10) + cost;
                IF c > c_temp THEN SET c = c_temp; END IF;
                SET c_temp = CONV(HEX(SUBSTRING(cv1, j+1, 1)), 16, 10) + 1;
                IF c > c_temp THEN SET c = c_temp; END IF;
                SET cv0 = CONCAT(cv0, UNHEX(HEX(c))), j = j + 1;
            END WHILE;
            SET cv1 = cv0, i = i + 1;
          END WHILE;
        END IF;
        RETURN c;
      END

To note:
    * Maximum length of input strings is 255 characters. The function should be edited to support more if needed.
    * I’ve tested it with international characters on a utf8_bin column and it seemed to work, but I’ve not tested that capability exstensively.
    * I’ve only tested it on MySQL 5.0+. No idea how it will work on versions less than that.