Working MySQL 5.1+ Levenshtein Stored Procedure
Posted by Kelvin on 13 Apr 2011 at 03:18 pm  Tagged as: programming
Update: Changed 0x00 to '\0' as per JanHendrik's comment below.
There are a number of MySQL functions for calculating Levenshtein distance floating around StackOverflow and other forums. They all seem to be based off http://codejanitor.com/wp/2007/02/10/levenshteindistanceasamysqlstoredfunction/ (broken link).
Anyway, I couldn't get them to work for me. MySQL complained:
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 4
Well, it turns out that you need to specify a delimiter instead of the default delimiter of ;. So here's a working version of the levenstein distance function, courtesy of CodeJanitor.
DELIMITER // 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 = '\0', 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//

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