Просечно трошење во последните 3 дена за цел систем

Просечно дневно трошење за последните три дена, земајќи ги предвид сите трансакции:

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)