SELECT Temp2.prozent as ansatz
FROM Temp2 INNER JOIN BV ON (BV.BKst=Temp2.kstktr) 
WHERE BV.monat+BV.jahr*12 BETWEEN Temp2.begPeriod AND Temp2.endPeriod;

Returns in version 4.15 and 4.14 1413 records

SELECT Temp2.prozent as ansatz
FROM Temp2 INNER JOIN BV ON (BV.BKst=Temp2.kstktr) 
WHERE BV.monat+BV.jahr*12 >= Temp2.begPeriod AND
BV.monat+BV.jahr*12 <= Temp2.endPeriod;

Returns in version 4.15 9891 records and 4.14 1413 records

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 Comments and Workarounds

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The problem was the constant '12' in the join expression. It was being evaluated once and then applied to all join conditions when it should be evaluated for every row individually.

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 Resolution

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Fixed Problem on 12/24/2004 in version 4.16 build 1