Просечно трошење во последните 3 дена за корисник

Просечно дневно трошење за последните 3 дена за одреден корисник:

CREATE OR REPLACE FUNCTION get_average_daily_spending_last_3_days_user(
    p_user_id INT
)
RETURNS TABLE (
    average_spending_last_3_days NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        SUM(daily_spending) / 3 AS average_spending_last_3_days
    FROM (
        SELECT
            d.day::date AS transaction_date,
            COALESCE(SUM(tb.spent_amount), 0) AS daily_spending
        FROM (
            SELECT CURRENT_DATE - 2 AS day
            UNION ALL
            SELECT CURRENT_DATE - 1
            UNION ALL
            SELECT CURRENT_DATE
        ) d
        LEFT JOIN transaction_account ta
            ON ta.user_id = p_user_id
        LEFT JOIN transaction_breakdown tb
            ON ta.transaction_account_id = tb.transaction_account_id
        LEFT JOIN transaction t
            ON t.transaction_id = tb.transaction_id
            AND t.date::date = d.day
        GROUP BY d.day
    ) daily_totals;
END;
$$;

Релациона алгебра

• TA(transaction_account_id, user_id)
• T(transaction_id, date)
• TB(transaction_id, spent_amount)
• D(day) -- генерирани последните 3 дена

Генерирање на последните 3 дена:

• D ← { CURRENT_DATE − 2, CURRENT_DATE − 1, CURRENT_DATE }

LEFT JOIN со сметка на корисник:

• J1 ← D ⟕_{TA.user_id = p_user_id} TA
• J2 ← J1 ⟕_{TA.transaction_account_id = TB.transaction_account_id} TB
• J3 ← J2 ⟕_{TB.transaction_id = T.transaction_id ∧ DATE(T.date) = D.day} T

Групирање по ден и пресметка на дневно трошење:

• G ← γ_{day; Σ(spent_amount) → daily_spending}(J3)

Просечно дневно трошење (фиксно делење со 3):

• R_final ← γ_{; (Σ(daily_spending) / 3) → average_spending_last_3_days}(G)