I want to generate random number between 1- 400,000 which is not the same value.

Example.
If X1 = 254899
X2 not have the same value as X1.
and X3 not have the same value as X1 and X2 .

I want to generate data X1 - X400,000 which not have the same value and each data have value between 1-400,000

If you know how to do that, please tell me. Thank you.