Well, there's no direct query I can think of but the following might help -

First, you need a unified set of results (I'm using MySQL syntax for ease of illustration)

SELECT columnA AS id FROM tableA
UNION
SELECT columnB AS id FROM tableB

Then, you need the range of numbers to limit your search for a new number

SELECT min(id), max(id) FROM
(FirstResults)

If you pulled your first results and put them into an array, you could get away without the 2nd DB query.

From there, I would use your programming language's array functions to create another array with all the possibilities between the min and max and compare the first array (all used id's) to the second array (all possible id's). In PHP, there is array_diff() to find the items in one array that are not in the other array. Then, select the lowest value in the array of differences.

Depending on the number of combined records in the 2 tables and the frequency of your need to perform this search, there are other approaches (like iterating through all the possibilities until you found one not in the first result set).

Hope this gets you through your dilemma!

Thanks Steve...the UNION statement and logic you gave above should do it! It's the first time in SQL where I need to select a number that was NOT available in a table. :-)

-Chris