In Lil, the readcsv[] function takes an optional string specifying a type for each column to decode:

    purchases:readcsv[read["purchases.csv"] "sii"]

Summing a column:

    sum purchases.amount

To create a summary, we need to reduce each group to a single row:

    select first country sum amount by country from purchases

Discounting:

    select first country sum amount-discount by country from purchases

Lil doesn't have a "median" primitive. Decks can contain multiple modules, but we happen to know this one is alone. Your path will vary:

    stats:first import["stats.deck"]
    select first country sum amount-discount by country where amount<stats.median[amount]*10 from purchases

Calculating the median within each group is merely a matter of reordering clauses:

    select first country sum amount-discount where amount<stats.median[amount]*10 by country from purchases