==== Вкупно потрошено во тековниот месец за цел систем
Вкупна сума на трошоци во тековниот месец
{{{#!sql
CREATE OR REPLACE FUNCTION get_total_spent_current_month()
RETURNS TABLE (
    total_spent NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        COALESCE(SUM(tb.spent_amount), 0) AS total_spent
    FROM transaction_breakdown tb
    JOIN transaction t
        ON tb.transaction_id = t.transaction_id
    WHERE EXTRACT(MONTH FROM t.date) = EXTRACT(MONTH FROM CURRENT_DATE)
      AND EXTRACT(YEAR FROM t.date) = EXTRACT(YEAR FROM CURRENT_DATE);
END;
$$;
}}}

==== Релациона алгебра
- TB(transaction_id, spent_amount)
- T(transaction_id, date)

JOIN на табелите:
- J1 ← TB ⨝,,TB.transaction_id = T.transaction_id,, T

Филтрирање на трансакции од тековниот месец и година:
- F ← σ,,EXTRACT(MONTH FROM date) = EXTRACT(MONTH FROM CURRENT_DATE) ∧ EXTRACT(YEAR FROM date) = EXTRACT(YEAR FROM CURRENT_DATE),,(J1)

Агрегација на вкупно потрошено:
- R_final ← γ,,; SUM(spent_amount) → total_spent,,(F)