Get top 10 products for every category

Question

I have a query which is something like this

SELECT 
t.category, 
tc.product, 
tc.sub-product,
 count(*) as sales 
 FROM tg t, ttc tc
 WHERE t.value = tc.valu         


        

Answer 1

There are probably reasons not to use analytical functions, but using analytical functions alone:

select am, rf, rfm, rownum_rf2, rownum_rfm
from
(
    -- the 3nd level takes the subproduct ranks, and for each equally ranked
    -- subproduct, it produces the product ranking
    select am, rf, rfm, rownum_rfm,
      row_number() over (partition by rownum_rfm order by rownum_rf) rownum_rf2
    from
    (
        -- the 2nd level ranks (without ties) the products within
        -- categories, and subproducts within products simultaneosly
        select am, rf, rfm,
          row_number() over (partition by am order by count_rf desc) rownum_rf,
          row_number() over (partition by am, rf order by count_rfm desc) rownum_rfm
        from
        (
            -- inner most query counts the records by subproduct
            -- using regular group-by. at the same time, it uses
            -- the analytical sum() over to get the counts by product
            select tg.am, ttc.rf, ttc.rfm,
              count(*) count_rfm,
              sum(count(*)) over (partition by tg.am, ttc.rf) count_rf
            from tg inner join ttc on tg.value = ttc.value
            group by tg.am, ttc.rf, ttc.rfm
        ) X
    ) Y
    -- at level 3, we drop all but the top 5 subproducts per product
    where rownum_rfm <= 5   -- top  5 subproducts
) Z
-- the filter on the final query retains only the top 10 products
where rownum_rf2 <= 10  -- top 10 products
order by am, rownum_rf2, rownum_rfm;

I used rownum instead of rank so you don't ever get ties, or in other words, ties will be randomly decided. This also doesn't work if the data is not dense enough (less than 5 subproducts in any of the top 10 products - it may show subproducts from some other products instead). But if the data is dense (large established database), the query should work fine.

---

The below makes two passes of the data, but returns correct results in each case. Again, this is a rank-without-ties query.

select am, rf, rfm, count_rf, count_rfm, rownum_rf, rownum_rfm
from
(
    -- next join the top 10 products to the data again to get
    -- the subproduct counts
    select tg.am, tg.rf, ttc.rfm, tg.count_rf, tg.rownum_rf, count(*) count_rfm,
        ROW_NUMBER() over (partition by tg.am, tg.rf order by 1 desc) rownum_rfm
    from (
        -- first rank all the products
        select tg.am, tg.value, ttc.rf, count(*) count_rf,
            ROW_NUMBER() over (order by 1 desc) rownum_rf
        from tg
        inner join ttc on tg.value = ttc.value
        group by tg.am, tg.value, ttc.rf
        order by count_rf desc
        ) tg
    inner join ttc on tg.value = ttc.value and tg.rf = ttc.rf
    -- filter the inner query for the top 10 products only
    where rownum_rf <= 10
    group by tg.am, tg.rf, ttc.rfm, tg.count_rf, tg.rownum_rf
) X
-- filter where the subproduct rank is in top 5
where rownum_rfm <= 5
order by am, rownum_rf, rownum_rfm;

columns:

count_rf : count of sales by product
count_rfm : count of sales by subproduct
rownum_rf : product rank within category (rownumber - without ties)
rownum_rfm : subproduct rank within product (without ties)