MYSQL: update field with random fixed char

This may sound rather evil but you might find this interesting

Suppose your table looked like this

CREATE TABLE massiveTable
(
    PK VARCHAR(50) NOT NULL,
    ... Other Fields ....
    PRIMARY KEY (PK)
) ENGINE=MyISAM;

Step 01 : Create a Empty Copy of the Table and another for Keys

CREATE TABLE randomKeys SELECT PK FROM massiveTable WHERE 1=2;
CREATE TABLE randomTable LIKE massiveTable;

Step 02 : Iteratively load every fifth key

SET @x = 0;
INSERT INTO randomKeys
SELECT PK FROM
(SELECT PK,(@x:=@x+1) num FROM massiveTable) A
WHERE MOD(num,5)=0;

Step 03 : Create the Random Temp Table

ALTER TABLE randomTable DISABLE KEYS;
INSERT INTO randomTable
SELECT B.* FROM randomKeys A LEFT JOIN randomTable B USING (PK);
ALTER TABLE randomTable ENABLE KEYS;

Step 04 : There is no Step 04. That's it !!! No stored procedure needed.

CAVEAT : I cannot make any promises on the running time or query processing.

Give it a Try !!!

Your post piqued my curiosity and I look at the RAND() function - which you can't perform GROUP BYs with. I found this and if your table has a PRIMARY KEY, there is a MySQL pseudo-column (_rowid), much like is found in Oracle, Firebird (and maybe others) then you can issue queries such as this one (see structure and data for comments table (sample) below).

select MOD(_rowid, 3) AS my_field, COUNT(ticket_id) 
FROM comments 
GROUP BY my_field;

+----------+------------------+
| my_field | count(ticket_id) |
+----------+------------------+
|        0 |                6 |
|        1 |                7 |
|        2 |                7 |
+----------+------------------+

Now, if your PK is some sort of AUTO INCREMENT and "random" with respect to your data, you could perhaps use the _rowid pseudo-column (with MOD) to generate a sufficient degree of randomness for your requirements, and perform your aggregate queries?

---- structure and data in comments table---

CREATE TABLE `comments` 
(
  `comment_id` int(11) NOT NULL,
  `ticket_id` int(11) NOT NULL,
  PRIMARY KEY (`comment_id`)
);


mysql> SELECT * FROM comments;
+------------+-----------+
| comment_id | ticket_id |
+------------+-----------+
|          1 |         1 |
|          2 |         1 |
|          3 |         1 |
|          4 |         1 |
|          5 |         2 |
|          6 |         2 |
|          7 |         2 |
|          8 |         2 |
|          9 |         3 |
|         10 |         3 |
|         11 |         3 |
|         12 |         3 |
|         13 |         4 |
|         14 |         4 |
|         15 |         4 |
|         16 |         4 |
|         17 |         5 |
|         18 |         5 |
|         19 |         5 |
|         20 |         5 |
+------------+-----------+