HowTo Sample a Certain amount of records

Question

Ok, I have created a table that has the top customers from 10 stores, so there is a Store Number column like 1, 2, 3.... and there is also a aggregated sum() Store Spend column that shows how much the customer spent in the last_12_months.
I now would like to sample the best/top 500 records from each of the stores, using Descending order and the spend column last_12_months column......
How would I do something like that ?
Thanks!

Answer 1

Hi,
That's an example of a "Top-N" query. You want to pick N items (N=500) from the top of a sorted list (or, in this case, 10 sorted lists - a separate list for each store).
In a sub-query, get all the columns you need, applying whatever WHERE-clause is appropriate (such as "WHERE sales_dt >= ADD_MONTHS (SYSDATE, -12)").
Also select a column that numbers each row according to how it compares to other rows in its class.
For example, if each store_number is a different class, and the ranking is determined by SUM (spend) in descending order, the new column might be
,       RANK () OVER
            ( PARTITION BY  store_number
              ORDER BY      SUM (spend)    DESC
            )  AS rnkThen, in your main query, choose only the rows "WHERE rnk <= 500".
The sub-query is necessary because analytic functions (like RANK) are evaluated after the WHERE-clause, but you want the opposite. You need to compute rnk and then use it in a WHERE-clause.
If you need more help, post some sample data and the results you want from that data.
You'll probably want to change your requirements to accomodate a small data set, such as picking the top 3 per store, rather than the top 500.