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Editorial Team
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Asked: 2026-05-26T06:19:25+00:00

In my db application I have a requirement for a unique, 4-digit number field

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In my db application I have a requirement for a unique, 4-digit number field for each customer. Up until 9999 I can just use autoincrements, but after that I will have to reuse numbers of customers that have been deleted (there won’t be more than 5000 customers at a given time but there may be more than 9999 customers over the lifetime of the system).

Question 1: Is there a (My)SQL statement to find the next reusable free number?

Question 2: If I get the number, assign it to a new customer and save the customer all in one transaction, similar transactions taking place at the same time will be sequentialized by the database so the numbers won’t collide, right?

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1 Answer

  1. Editorial Team

    You’d be better off storing a table with all 10,000 possible values defined, and an “in-use” flag on each. That way, releasing the number for re-use is a simple update to set “inuse=false”.

    Also makes finding the lowest available value a simple

    SELECT idstring
FROM idstringtable
ORDER BY idstring ASC
WHERE (available = 1)
LIMIT 1

    Doing that with appropriate locks/transactions would prevent two or more requests getting the same ID, and since it’s a small table, doing a global table lock would not significantly impact performance.

    Otherwise, you’d be stuck rummaging around your users table, trying to find the first “gap” in the numbering sequence.

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