Unwalking a string with GROUP_CONCAT

“Walking a string” is an SQL technique to convert a single value into multiple rows result set. For example, walking the string ‘hello’ results with 5 rows, each of which contains a single character from the text.

I’ll present a brief example of walking a string, and then show how to “unwalk” the string: do the reverse operation.

To walk a string, an integers table is required (or this could be a good use for SeqEngine):

CREATE TABLE `int_table` (
  `int_col` int(11) NOT NULL,
  PRIMARY KEY  (`int_col`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

-- ...
-- INSERTS follow here
-- ...

mysql> SELECT * FROM int_table;
+---------+
| int_col |
+---------+
|       0 |
|       1 |
|       2 |
|       3 |
|       4 |
|       5 |
|       6 |
|       7 |
|       8 |
|       9 |
+---------+
10 rows in set (0.00 sec)

To convert a string to rows of characters, we join the text with the integers table (we assume there are enough numbers for covering the length of the text):

mysql> SELECT
         SUBSTRING(s, int_col+1, 1) AS c
       FROM int_table, (SELECT 'hello' AS s) sel1
       WHERE int_col < char_length(s);
+---+
| c |
+---+
| h |
| e |
| l |
| l |
| o |
+---+
5 rows in set (0.00 sec)

More on this can be found in the excellent SQL Cookbook.

Unwalking the string

Doing the inverse action – combining the string back from the multiple rows, can be easily done using GROUP_CONCAT. It’s interesting to learn that GROUP_CONCAT does not actually require any GROUP BY clause. When no such clause is provided in the SQL query, all searched rows are used.

Let’s assume now that we have a table of character values, which we want to concatenate back to a complete string. We can easily build this table:

CREATE TABLE characters AS
  SELECT
    SUBSTRING(s, int_col+1, 1) AS c
  FROM int_table, (SELECT 'hello' AS s) sel1
  WHERE int_col < char_length(s);

To reconstruct the text, we simply use MySQL’s GROUP_CONCAT with an empty separator:

mysql> SELECT GROUP_CONCAT(c separator '') AS s FROM characters;
+-------+
| s     |
+-------+
| hello |
+-------+
1 row in set (0.00 sec)