Trying to understand over() and partition by

Question

I am trying to get the over and partition by functionality wrapped around my head. Here is an example that I just do not understand.

Here is the data I have:

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Answer 1

The partition by orderdate means that you're only comparing records to other records with the same orderdate. For example, of the five records with orderdate = '08/01/2001', one will have row_number() = 1, one will have row_number() = 2, and so on.

The order by orderdate asc means that, within a partition, row-numbers are to be assigned in order of orderdate. In your example that has no effect, because you're already partitioning by orderdate, so all records within a partition will have the same orderdate. (It would be like writing SELECT ... FROM t WHERE c = 6 ORDER BY c: all selected records have the same value of c, so the ORDER BY c does nothing.) So, within a partition, the assignment of row_number() is arbitrary: each row will have a different number, but there are no guarantees about which row will have which number.