smart way to generate unique random number

Question

i want to generate a sequence of unique random numbers in the range of 00000001 to 99999999.

So the first one might be 00001010, the second 40002928 etc.

The easy way is to generate a random number and store it in the database, and every next time do it again and check in the database if the number already exists and if so, generate a new one, check it again, etc. But that doesn't look right, i could be regenerating a number maybe 100 times if the number of generated items gets large.

Is there a smarter way?

EDIT as allways i forgot to say WHY i wanted this, and it will probably make things clearer and maybe get an alternative, and it is: we want to generate an ordernumber for a booking, so we could just use 000001, 000002 etc. But we don't want to give the competitors a clue of how much orders are created (because it's not a high volume market, and we don't want them to know if we are on order 30 after 2 months or at order 100. So we want to have an order number which is random (yet unique)

Answer 1

Using this algorithm might be suitable, though it's memory consuming: http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle Put the numbers in the array from 1 to 99999999 and do the shuffle.

Answer 2

For the extremely limited size of your numbers no you cannot expect uniqueness for any type of random generation.

You are generating a 32bit integer, whereas to reach uniqueness you need a much larger number in terms around 128bit which is the size GUIDs use which are guaranteed to always be globally unique.