Best way to deal with misspellings in a MySQL fulltext search

Question

I have about 2000 rows in a mysql database.

Each row is a max of 300 characters and contains a sentence or two.

I use mysql's built in fulltext search to search these rows.

I would like to add a feature so that typos and accidental mispellings are corrected, if possible.

For example, if someone types "right shlder" into the searchbox, this would equate to "right shoulder" when performing the search.

What are your suggestions on the simplest way to add this kind of functionality? Is it worth adding an external search engine of some kind, like lucene? (It seems like for such a small dataset, this is overkill.) Or is there a simpler way?

Answer 1

I think you should use SOUNDS LIKE or SOUNDEX()

As your data set is so small, one solution may be to create a new table to store the individual words or soundex values contained in each text field and use SOUNDS LIKE on that table.

eg:

SELECT * FROM table where id IN 
(
    SELECT refid FROM tableofwords 
    WHERE column SOUNDS LIKE 'right' OR column SOUNDS LIKE 'shlder'
)

see: http://dev.mysql.com/doc/refman/5.0/en/string-functions.html

I belive it is not possible to wild card seach the string :(

Answer 2

MySQL doesn't support SOUNDEX search in fulltext.

If you want to implemente a lucene like framework, it means that you have to take all the documents, splits them into words, and then builds an index for each word.

When someone search for "right shlder" you have to make a SOUNDEX search for each words in the worlds table:

    $search = 'right shlder';
preg_match_all('(\w+)', $search, $matches);
if (!empty($matches[0]))
   $sounds = array_map('soundex', $matches[0]);
$query = 'SELECT word FROM words_list
    WHERE SOUNDEX(word) IN(\''.join('\',\'',$sounds).'\')';

and then make a fulltext search:

$query2 = 'SELECT * FROM table
    WHERE MATCH(fultextcolumn)
    AGAINST ('.join (' OR ', $resuls).' IN BINARY MODE)';

Where $result is an array with the results of the first query.

Answer 3

The technical term for what you are looking for, is Levenshtein distance which is used to calculate the difference between two sequences (in this case a sequence of characters which is a string).

PHP actually has two built in function for that, the first being similar_text and the other called levenshtein which should help you out with your problem. You will have to benchmark if it is fast enough for your needs.

Answer 4

We had the same problem and made 2 relatively fast stored procedures. Levenshtein distance is good but not great for autocompleting searching "catema" will match quite well with "catamaran" even though it has a bad Levenshtein distance. Making our function well suited for type as you go searches.

We have 2 versions, 1 is optimized to work with a big indexed word table and uses the first letter to narrow down the search for significant performance gain.

SELECT fuzzy_match_first_word('catema', `word`, 80) FROM `dictionary` WHERE (`word` LIKE 'c%') AND (fuzzy_match_first_word('catema', `word`, 80)>=80)

The other version will word on bigger strings, not just comparing single words.

SELECT fuzzy_match('catema', `subject`, 80) FROM `dictionary` WHERE (fuzzy_match('catema', `subject`, 80)>=80)

Stored Procedures:

DELIMITER //
CREATE OR REPLACE FUNCTION `fuzzy_match_first_word`(`str_needle` VARCHAR(64), `str_haystack` VARCHAR(4096), `minimum_quality` INT(11)) RETURNS INT(11)
    DETERMINISTIC
BEGIN
    DECLARE needleLen, haystackLen, iIdx, cLen, mLen, penalty, checkSpan, shiftAmount INT DEFAULT 0;
    DECLARE sChar, subCharNeedle CHAR(1) DEFAULT ' ';
    DECLARE res INT DEFAULT 100;
    DECLARE n INT DEFAULT 2; -- assume first letter to be ok, needs to be checked by outer like on indexed field
    DECLARE shifted INT DEFAULT 4; -- how often we allow letters being moved
    SET needleLen   = CHAR_LENGTH(str_needle);
    SET haystackLen = CHAR_LENGTH(str_haystack);
    SET checkSpan   = 2;                          -- Check_span decides how wide to check - Min: 1, Max: Not sensible beyond 5.
    IF (needleLen < 1) OR (haystackLen < 1) THEN SET res = 0; ELSE
        SET sChar= LEFT(str_needle,1);
        IF (haystackLen <= needleLen) THEN
            SET cLen = haystackLen;
            SET res = res-(20*(needleLen-haystackLen)); -- 30 penalty for each missing letter
            if(res < minimum_quality) THEN RETURN 0; END IF;
            SET mLen = cLen;
        ELSE
            SET cLen = needleLen;
            SET mLen = haystackLen;
        END IF;
        WHILE n <= cLen DO
                SET subCharNeedle = SUBSTRING(str_needle, n, 1);
                IF(SUBSTRING(str_haystack, n + shiftAmount, 1) <> subCharNeedle) THEN
                    `fail_check`:
                    BEGIN -- check if not correct
                    SET penalty = 20; -- 20% reduction for each missed letter, 5% for closeness a close hit
                    FOR i IN 1..checkSpan DO
                    -- positive (assume missing letter more likely than a added letter)
                    SET iIdx = (n + i);
                    IF (iIdx > 0) AND (iIdx <= mLen) THEN
                        IF (SUBSTRING(str_haystack, iIdx + shiftAmount, 1) = subCharNeedle) THEN
                            SET penalty = 5*i;
                            IF shifted > 0 THEN
                                SET shifted = shifted-1;
                                SET shiftAmount = i + shiftAmount;
                            END IF;
                            LEAVE `fail_check`;
                        END IF;
                    END IF;
                    -- negative
                    SET iIdx = (n - i);
                    IF (iIdx > 0) AND (iIdx <= mLen) THEN
                        IF (SUBSTRING(str_haystack, iIdx + shiftAmount, 1) = subCharNeedle) THEN
                            SET penalty = 5*i;
                            IF shifted > 0 THEN
                                SET shifted = shifted-1;
                                SET shiftAmount = -i + shiftAmount;
                            END IF;
                            LEAVE `fail_check`;
                        END IF;
                    END IF;
                    END FOR;
                END; -- end of fail_check
                SET res = res - penalty;
                if(res < minimum_quality) THEN RETURN 0; END IF;
            END IF;
        SET n = n + 1;
    END WHILE;
END IF;
RETURN res;
END //
DELIMITER ;


DELIMITER //

CREATE OR REPLACE FUNCTION fuzzy_match(str_needle VARCHAR(64), str_haystack VARCHAR(4096), minimum_quality INT)
    RETURNS INT DETERMINISTIC CONTAINS SQL

BEGIN
    DECLARE needle_len, haystack_len, cIdx, iIdx, cLen, loop_abort, n INT DEFAULT 0;
    DECLARE sub_len, check_span INT;
    DECLARE sSub VARCHAR(4096);
    DECLARE sChar, subChar_needle, subChar_tmp CHAR(1) DEFAULT ' ';
    DECLARE res, rmatch_score, minq FLOAT DEFAULT 0;
    SET str_needle   = UPPER(REPLACE(TRIM(str_needle),' ',''));
    SET str_haystack = UPPER(REPLACE(TRIM(str_haystack),' ',''));
    SET needle_len   = CHAR_LENGTH(str_needle);
    SET haystack_len = CHAR_LENGTH(str_haystack);
    SET minq = (minimum_quality / 100.0);
    SET check_span   = 2;                          -- Check_span decides how wide to check - Min: 1, Max: Not sensible beyond 5.
    SET sChar= LEFT(str_needle,1);
    IF (needle_len > 0) AND (haystack_len > 0) THEN
        REPEAT
            SET cIdx = IFNULL(LOCATE(sChar, str_haystack, cIdx+1), 0);
            IF (cIdx > 0) THEN
                SET sSub = SUBSTRING(str_haystack, cIdx, needle_len+1);
                SET cLen = CHAR_LENGTH(sSub);
                SET sub_len = CHAR_LENGTH(sSub);
                SET cLen = (sub_len * (sub_len < needle_len)) + (needle_len * (sub_len >= needle_len));
                SET rmatch_score = 0;
                WHILE (loop_abort = 0) AND  (n < cLen) DO
                        SET n = n + 1;
                        SET subChar_needle = SUBSTRING(str_needle, n, 1);
                        IF (subChar_tmp <> subChar_needle) THEN
                            SET subChar_tmp = subChar_needle;
                            FOR i IN -check_span..check_span DO
                            SET iIdx = (n + i - 1);
                            IF (iIdx >= 0) AND (iIdx < cLen) THEN
                                IF (subChar_needle = SUBSTRING(sSub, iIdx + 1, 1)) THEN
                                    SET rmatch_score = rmatch_score + (check_span + 1 - ABS(i));
                                END IF;
                            END IF;
                        END FOR;
                        SET loop_abort = ((rmatch_score / (check_span * n)) < minq);
                        ELSE
                        SET rmatch_score = rmatch_score + check_span;
                    END IF;
            END WHILE;
            SET res = (rmatch_score / ((check_span + 1) * needle_len));
        END IF;
        UNTIL (cIdx <= 0) OR (res >= 1) END REPEAT;
    END IF;
    RETURN (res >= minq) * ROUND(res * 100);
END //
DELIMITER ;