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.