Levenshtein: MySQL + PHP

Question

You need a levenshtein function in MySQL and query like

$word = mysql_real_escape_string($word);
mysql_qery("SELECT `term` FROM `words` WHERE levenshtein('$word', `term`) BETWEEN 0 AND 4");

There are two ways to implement a Levenshtein function in MySQL. The first is to create a STORED FUNCTION which operates much like a STORED TRANSACTION, except it has distinct inputs and an output. This is fine for small datasets, but a little slow on anything approaching several thousand rows.

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;
-- max strlen=255
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//

Store the above code in a .sql file and import it into your database like so:

source /tmp/mysql_udf.sql

The second method is to implement a User Defined Function in C/C++ and link it into MySQL as a shared library (*.so file). This method also uses a STORED FUNCTION to call the library, which means the actual query for this or the first method may be identical (providing the inputs to both functions are the same). You can find out more about this method here: http://samjlevy.com/mysql-levenshtein-and-damerau-levenshtein-udfs/

With either of these methods, your query would be something like:

SELECT term FROM words WHERE levenshtein(term, 'term') < 5;

Also, remember that the 'threshold' value should change in relation to the original word length. It's better to think of it in terms of a percentage value, i.e. half your word = 50%, half of 'term' = 2.

If you have a huge database, you can filter the words first using SOUNDEX:

$word = strtolower(mysql_real_escape_string($_GET['term']));

$rs = mysql_query("SELECT LOWER(`term`) FROM `words` WHERE SOUNDEX(term) = SOUNDEX(" . $word . ")");

while ($row = mysql_fetch_assoc($rs)) { 

    $lev = levenshtein($word, $row['term']);

    ....

}

If you have time enough to play with a C extension or procedure, you may achieve better performance, but filtering the records on mysql before applying real levenshtein will make things faster with almost no effort.

If you are dealing with very large data sets I have found that it is much more efficient to handle the Levenshtein operations and sorting in PHP than it is in MySQL. e.g. query of about 1000 records:

MySQL( ~ 0.0050s) -> PHP Levenshtein( ~ 1.300s)

vs.

MySQL Levenshtein( >= 5.000s) -> PHP( ~ 0.250s)

There are also many other options for optimizing search engines but if you want to use Levenshtein just be aware of the data you'll be handling and the latencies you want.

You can make this code look a bit neater but @profitphp is right, you can't doing it in MySQL without a levenstein library.

$word = strtolower($_GET['term']);

$q = mysql_uqery("SELECT LOWER(`term`) FROM `words`");

while($r = mysql_fetch_assoc($q)) { 

    $lev = levenshtein($word, $r['term']);

    ....

}

I suggest you including the call of the levenshtein(link: http://www.artfulsoftware.com/infotree/queries.php#552) into your query.

You should use mysqli_query($q) because mysql_query($q) is deprecated and may be removed in future php versions!

$word = mysql_real_escape_string($word);
$query = "SELECT `term` FROM `words` WHERE levenshtein('$word', `term`)   BETWEEN 0 AND 4";
mysqli_qery($query);