Hugo Kornelis (4/15/2015)
But note that supplying an offset only produces the expected results if you can be sure that there is exactly one row per month in the sales table. (Or, if you use the PARTITION BY clause, exactly one row per distinct set of values of the partition columns and per month).LAG simply counts rows, it does not look at the actual values.
I think that the wording of the question makes it quite clear that one is looking at something that delivers the sales for a month - after all, it's made absolutely clear that that's what is being compared. What makes you think that the that it is not sensible to assume (given the wording of the question) that the query refers to a table rather than to some row set delivered by various joins, aggregates, and restrictions which result in monthly data?
Tom