Долг на кредитна картичка од минатиот месец за цел систем

Вкупна сума потрошена од сметките обележани како „кредитна картичка“ за минатиот месец

CREATE OR REPLACE FUNCTION get_credit_card_debt_last_month()
RETURNS TABLE (
    credit_card_debt NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        COALESCE(SUM(tb.spent_amount), 0) AS credit_card_debt
    FROM transaction_breakdown tb
    JOIN transaction_account ta
        ON tb.transaction_account_id = ta.transaction_account_id
    JOIN transaction t
        ON tb.transaction_id = t.transaction_id
    WHERE (ta.account_name ILIKE '%кредитна%' OR ta.account_name ILIKE '%credit%')
      AND EXTRACT(MONTH FROM t.date) = EXTRACT(MONTH FROM CURRENT_DATE - INTERVAL '1 MONTH')
      AND EXTRACT(YEAR FROM t.date) = EXTRACT(YEAR FROM CURRENT_DATE - INTERVAL '1 MONTH');
END;

Релациона алгебра

• TA(transaction_account_id, account_name)
• TB(transaction_id, transaction_account_id, spent_amount)
• T(transaction_id, date)

JOIN на сите табели:

• J1 ← TA ⨝_{TA.transaction_account_id = TB.transaction_account_id} TB
• J2 ← J1 ⨝_{TB.transaction_id = T.transaction_id} T

Филтрирање на кредитни картички и минат месец:

• F ← σ_{(account_name ILIKE '%кредитна%' OR account_name ILIKE '%credit%') ∧ EXTRACT(MONTH FROM date) = EXTRACT(MONTH FROM CURRENT_DATE - INTERVAL '1 MONTH') ∧ EXTRACT(YEAR FROM date) = EXTRACT(YEAR FROM CURRENT_DATE - INTERVAL '1 MONTH')}(J2)

Агрегација на вкупен долг:

• R_final ← γ_{; SUM(spent_amount) → credit_card_debt}(F)