==== Просечно трошење во последните 3 дена за цел систем
Просечно дневно трошење за последните три дена, земајќи ги предвид сите трансакции:
{{{#!sql
CREATE OR REPLACE FUNCTION get_average_daily_spending_last_3_days_fixed()
RETURNS TABLE (
    average_spending_last_3_days NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        SUM(daily_spending) / 3 AS average_spending_last_3_days
    FROM (
        SELECT
            d.day::date AS transaction_date,
            COALESCE(SUM(tb.spent_amount), 0) AS daily_spending
        FROM (
            SELECT CURRENT_DATE - 2 AS day
            UNION ALL
            SELECT CURRENT_DATE - 1
            UNION ALL
            SELECT CURRENT_DATE
        ) d
        LEFT JOIN transaction t
            ON t.date::date = d.day
        LEFT JOIN transaction_breakdown tb
            ON t.transaction_id = tb.transaction_id
        GROUP BY d.day
    ) daily_totals;
END;
$$;
}}}

==== Релациона алгебра
- T(transaction_id, date)
- TB(transaction_id, spent_amount)
- D(day)   -- генерирана релација со последните 3 дена

Генерирање на последните 3 дена:
- D ← { CURRENT_DATE − 2, CURRENT_DATE − 1, CURRENT_DATE }

LEFT JOIN со трансакции:
- J1 ← D ⟕,,DATE(T.date) = D.day,, T
- J2 ← J1 ⟕,,T.transaction_id = TB.transaction_id,, TB

Групирање по ден и пресметка на дневно трошење:
- G ← γ,,day; Σ(spent_amount) → daily_spending,,(J2)

Пресметка на просек (фиксно делење со 3):
- R_final ← γ,,; (Σ(daily_spending) / 3) → average_spending_last_3_days,,(G)