Generate sequence of alphabets in using oracle SQL query

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Oracle

Dear All,

I have got a task in which I have to generate alphabet sequence on execution of the same oracle query every time.

Example:
When I execute a query first time, it has to generate A
When I execute the same query second time, it has to generate B.

So, it should generate A,B,C,D......Z

Once it reaches Z, it has to generate AA,AB,AC.....AZ

Kindly help me in composing a query or any suggestion would be appreciated.

Regards,
Raj

What I have tried:

I have tried sequence generator by searching in Google but it is generating the sequence of all alphabets at the same time. But I need one by one alphabet on execution of the same query..