Best way for Unique Random String for MySQL Long table - mysql

Question

I know how to create random chars both at PHP and MySQL but the question is that I have to create a 4 char random string for a table of 10 thousand or so rows. What way is the best to make sure it will remain unique?
I can use a longer string if I need to but not longer then 12.
Just to make it simple, table exists I need to add an extra column and fill it with a 4 char random string and keys must remain unique.

Answer 1

An option:
Put all you possible characters in a table with only one column.
val
------
0
1
...
9
a
b
...
z
Use this query
SELECT CONCAT(a.val,b.val,c.val,d.val)
FROM chars AS a
JOIN chars AS b
JOIN chars AS c
JOIN chars AS d
ORDER BY RAND()
LIMIT 10000
On the other hand if you need to get one ID at a time I see two approaches.
A. If you have a lot of unassigned IDs available.
In this case you just generate an ID and see if it's free. If not try another one.
B. If you want to keep you assigned IDs and the available IDs in the same magnitude level.
In this case it would be best to pre-generate all your IDs, shuffle them, and when you need one just pick the next available one. Say put them all in a table, and when you assign one from that table, you remove it so it can't be picked again.
If your allowed characters are 0-9a-z this means the table will occupy 364. That's just a couple of MB.

Answer 2

As those strings need to be unique, why not use a numeric auto-increment value and then convert that to a character based value similar to the conversion of decimal to hex.
If you choose the e.g. all characters and digits you simply need to create a routine that will convert an integer to a "base 62" number.

Answer 3

You can make use of the DISTINCT keyword.
For example, the following query will only return unique rows by which you can validate that your 4 char random string remains unique:
mysql> SELECT DISTINCT random_strings FROM chars;

Answer 4

This may be lengthy, but would allow you to create what you need:
CREATE FUNCTION gen_alphanum () RETURNS CHAR(4)
RETURN
ELT(FLOOR(1 + (RAND() * (50-1))), 'a','b','c','d','e','f','g','h','i','j','k','l','m ','n','o','p','q','r','s','t','u','v','w','x','y', 'z',
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y', 'Z',);
)

Answer 5

It sounds like you've got the code in MySQL for creating these random valued strings.
Consider this option:
create a User Defined Function in MySQL. Have this function run the SQL statements to generate and return this new random string. Ensure that you use NOT EXISTS(SELECT MyRandomString FROM MyTable) within that creation statement to check that the random string doesn't already exist in the table.
When inserting new rows, use this function's return value to assign to the MyRandomString column.
to update the data existing, simply:
UPDATE MyTable
SET MyRandomString = fn_CreateSomeRandomString()
when inserting:
INSERT INTO MyTable (foo, bar, MyRandomString)
VALUES ('','', fn_CreateSomeRandomString());
Here's a sample of that UDF on PasteBin.

Answer 6

If you have MySQL 5.6, you can use TO_BASE64 as follows:
select LEFT( TO_BASE64( SHA(rand()) ), 6 ) ;
Alternatively if you don't have 5.6,
DELIMITER //
drop function if exists randChr //
create function randChr()
returns char
BEGIN
IF RAND() <= 0.5 THEN -- Lowercase
return CHAR( 97 + 25*rand() ) ;
ELSE -- uc
return CHAR( 65 + 25*rand() ) ;
END IF;
END //
drop function if exists randString //
create function randString( len int )
returns varchar(255)
BEGIN
SET #n = 0;
SET #res = '' ;
REPEAT
SET #res = concat( #res, randChr() ) ;
set #n = #n + 1 ;
UNTIL #n >= len END REPEAT;
return #res ;
END //
DELIMITER ;
-- USE:
select randString( 5 );
select randString( 60 );