>If these barcodes are to be used for commercial trade, you can't just pick any number since the number also contains a control digit.

So he can omit that digit, which then complicates the whole thing - there'd have to be a version of each code, one digit shorter, which would then be accounted for.

>Other than that, this code will find "holes" in a series
>
>

Select k.barcode+1 as missing From yourtable k;
>   Where k.barcode+1 Not in (Select barcode From yourtable)

It will find the beginning of each hole, however it won't find the size of the hole. IOW, if 20 consecutive numbers are missing, it will find the first one. But then with a fox cursor, index and Set Near ON it shouldn't be a problem to find the next existing number and gauge the size of the hole.