>Thomas,
>
>> I usually solve this kind of problem with recusive CTEs providing a
>> counter for the records returned, thus a "simple" select statement can
>> nicely filter on this kind of property without row numbers functionality.
>
>Can you provide an example, please.
>
>I, for one, can't imagine how a CTE could be used for such a purpose.

My imagination may be slightly more vivid than yours, Sean, I even think I answered something on this list with a similar recursive CTE a few years ago.

WITH RECURSIVE TMP(Level, PK, <fields>) as
(SELECT 1, min(PK), <whatever>
FROM <table>
WHERE <whatever>
UNION ALL
SELECT t.Level + 1, t1.PK, <fields>
FROM TMP t
JOIN <table> t1 on t.PK < t1.PK
LEFT JOIN <table> t2 on t2.PK > TMP.PK and t2.PK < t1.PK
WHERE <whatever> and t2.PK is null)
SELECT *
FROM TMP
WHERE MOD(Level, 5) = 0

This has the severe drawback that it doesn't work if there's more than 1024 records (I think, at least there's a very limited max depth of recursion), I consider it less intuitive than EXECUTE BLOCK, and expect it to be slower. But I agree that it would be interesting to see Thomas' solution.

Don't know whether windowing functions can be used in the WHERE clause, but when Firebird 3 is released, it would be tempting to try things like

WHERE MOD(ROW_NUMBER() OVER (ORDER BY <something>), 5) = 2

Set