Create Unique user names for a large amount of users

Hi.

I have a table which contains about 20,000 record of user information. The records include a user name and password column which I need to create. I also have a temp table (with the same structure) to which I insert records from a flat file.

I would like to be able to create user names which are unique both in the table and the temp table together.

So basically the probelm comes down to creating a unique user names for about 20,000 records. I would also like the user name to consist of 6 digits.

I've tried code to do this, but it generates only about 700 unique records and then fails. It is also not very efficient.

So if you have code/example/link which show how to do this, please help,

Thanks in advance.
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