mysql - Count number of unique characters in a string

Question

I'm looking for a sql statement to count the number of unique characters in a string.

e.g.

3333333333 -> returns 1
1113333333 -> returns 2
1112222444 -> returns 3

I did some tests with REGEX and mysql-string-functions, but I didn't find a solution.

Answer 1

This is for fun right?

SQL is all about processing sets of rows, so if we can convert a 'word' into a set of characters as rows then we can use the 'group' functions to do useful stuff.

Using a 'relational database engine' to do simple character manipulation feels wrong. Still, is it possible to answer your question with just SQL? Yes it is...

Now, i always have a table that has one integer column that has about 500 rows in it that has the ascending sequence 1 .. 500. It is called 'integerseries'. It is a really small table that used a lot so it gets cached in memory. It is designed to replace the from 'select 1 ... union ... text in queries.

It is useful for generating sequential rows (a table) of anything that you can calculate that is based on a integer by using it in a cross join (also any inner join). I use it for generating days for a year, parsing comma delimited strings etc.

Now, the sql mid function can be used to return the character at a given position. By using the 'integerseries' table i can 'easily' convert a 'word' into a characters table with one row per character. Then use the 'group' functions...

SET @word='Hello World';

SELECT charAtIdx, COUNT(charAtIdx)
FROM (SELECT charIdx.id,
    MID(@word, charIdx.id, 1) AS charAtIdx 
    FROM integerseries AS charIdx
    WHERE charIdx.id <= LENGTH(@word)
    ORDER BY charIdx.id ASC
    ) wordLetters
GROUP BY
   wordLetters.charAtIdx
ORDER BY charAtIdx ASC  

Output:

charAtIdx  count(charAtIdx)  
---------  ------------------
                            1
d                           1
e                           1
H                           1
l                           3
o                           2
r                           1
W                           1

Note: The number of rows in the output is the number of different characters in the string. So, if the number of output rows is counted then the number of 'different letters' will be known.

This observation is used in the final query.

The final query:

The interesting point here is to move the 'integerseries' 'cross join' restrictions (1 .. length(word)) into the actual 'join' rather than do it in the where clause. This provides the optimizer with clues as to how to restrict the data produced when doing the join.

SELECT 
   wordLetterCounts.wordId,
   wordLetterCounts.word,   
   COUNT(wordLetterCounts.wordId) AS letterCount
FROM 
     (SELECT words.id AS wordId,
             words.word AS word,
             iseq.id AS charPos,
             MID(words.word, iseq.id, 1) AS charAtPos,
             COUNT(MID(words.word, iseq.id, 1)) AS charAtPosCount
     FROM
          words
          JOIN integerseries AS iseq
               ON iseq.id BETWEEN 1 AND words.wordlen 
      GROUP BY
            words.id,
            MID(words.word, iseq.id, 1)
      ) AS wordLetterCounts
GROUP BY
   wordLetterCounts.wordId  

Output:

wordId  word                  letterCount  
------  --------------------  -------------
     1  3333333333                        1
     2  1113333333                        2
     3  1112222444                        3
     4  Hello World                       8
     5  funny - not so much?             13

Word Table and Data:

CREATE TABLE `words` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `word` varchar(128) COLLATE utf8mb4_unicode_ci NOT NULL,
  `wordlen` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

/*Data for the table `words` */

insert  into `words`(`id`,`word`,`wordlen`) values (1,'3333333333',10);
insert  into `words`(`id`,`word`,`wordlen`) values (2,'1113333333',10);
insert  into `words`(`id`,`word`,`wordlen`) values (3,'1112222444',10);
insert  into `words`(`id`,`word`,`wordlen`) values (4,'Hello World',11);
insert  into `words`(`id`,`word`,`wordlen`) values (5,'funny - not so much?',20);

Integerseries table: range 1 .. 30 for this example.

CREATE TABLE `integerseries` (
  `id` int(11) unsigned NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=500 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci