==== Долг на кредитна картичка од минатиот месец за корисник
Потрошено од сметки означени како „кредитна картичка“ за минатиот месец за еден корисник
{{{#!sql
CREATE OR REPLACE FUNCTION get_credit_card_debt_last_month_user(
    p_user_id INT
)
RETURNS TABLE (
    credit_card_debt NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        COALESCE(SUM(tb.spent_amount), 0) AS credit_card_debt
    FROM transaction_breakdown tb
    JOIN transaction_account ta
        ON tb.transaction_account_id = ta.transaction_account_id
    JOIN transaction t
        ON tb.transaction_id = t.transaction_id
    WHERE ta.user_id = p_user_id
      AND (ta.account_name ILIKE '%кредитна%' OR ta.account_name ILIKE '%credit%')
      AND EXTRACT(MONTH FROM t.date) = EXTRACT(MONTH FROM CURRENT_DATE - INTERVAL '1 MONTH')
      AND EXTRACT(YEAR FROM t.date) = EXTRACT(YEAR FROM CURRENT_DATE - INTERVAL '1 MONTH');
END;
$$;
}}}

==== Релациона алгебра
- TA(transaction_account_id, user_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 по корисник, кредитна картичка и минат месец:
- F ← σ,,user_id = p_user_id ∧ (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)