How to make unique random alphanumeric sequence in SQL Server

Question

I want to make unique random alphanumeric sequence to be the primary key for a database table.

Each char in the sequence is either a letter (a-z) or number (0-9)

Answer 1

It is a very bad idea to use random strings as a primary key.
It will effect performance as well as storage size, and you will be much better of using an int or a bigint with an identity property.

However, generating a random string in SQL maybe useful for other things, and this is why I offer this solution:

Create a table to hold permitted char values.
In my example the permitted chars are 0-9 and A-Z.

CREATE TABLE Chars (C char(1))

DECLARE @i as int = 0
WHILE @i < 10
BEGIN
  INSERT INTO Chars (C) VALUES (CAST(@i as Char(1)))
  SET @i = @i+1
END

SET @i = 65
WHILE @i < 91
BEGIN
  INSERT INTO Chars (C) VALUES (CHAR(@i))
  SET @i = @i+1
END

Then use this simple select statement to generate a random string from this table:

SELECT TOP 10 C AS [text()]
FROM Chars
ORDER BY NEWID()
FOR XML PATH('')

The advantages:

• You can easily control the allowed characters.
• The generation of a new string is a simple select statement and not manipulation on strings.

The disadvantages:

• This select results with an ugly name (i.e XML_F52E2B61-18A1-11d1-B105-00805F49916B). This is easily solved by setting the result into a local variable.

• Characters will only appear once in every string. This can easily be solved by adding union:

example:

SELECT TOP 10 C AS [text()]
  FROM (
    SELECT * FROM Chars
    UNION ALL SELECT * FROM Chars
  ) InnerSelect
  ORDER BY NEWID()
  FOR XML PATH('')

Another option is to use STUFF function instead of As [Text()] to eliminate those pesky XML tags:

SELECT STUFF((
 SELECT TOP 100 ''+ C 
 FROM Chars
 ORDER BY NEWID()
 FOR XML PATH('')
), 1, 1, '') As RandomString;

This option doesn't have the disadvantage of the ugly column name, and can have an alias directly. Execution plan is a little different but it should not suffer a lot of performance lose.

Play with it yourself in this Sql Fiddle

If there are any more advantages / disadvantages you think of please leave a comment. Thanks.