==== Годишни трендови на трансакции за цел систем
Трошења според сметки за секој квартал од тековната година
{{{#!sql
CREATE OR REPLACE FUNCTION get_quarterly_spending_trends_system()
RETURNS TABLE (
    account_name TEXT,
    q1_spent NUMERIC,
    q2_spent NUMERIC,
    q3_spent NUMERIC,
    q4_spent NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        ta.account_name, 
        SUM(CASE WHEN EXTRACT(QUARTER FROM t.date) = 1 THEN tb.spent_amount ELSE 0 END) AS q1_spent,
        SUM(CASE WHEN EXTRACT(QUARTER FROM t.date) = 2 THEN tb.spent_amount ELSE 0 END) AS q2_spent,
        SUM(CASE WHEN EXTRACT(QUARTER FROM t.date) = 3 THEN tb.spent_amount ELSE 0 END) AS q3_spent,
        SUM(CASE WHEN EXTRACT(QUARTER FROM t.date) = 4 THEN tb.spent_amount ELSE 0 END) AS q4_spent
    FROM transaction_breakdown tb
    JOIN transaction t 
        ON tb.transaction_id = t.transaction_id
    JOIN transaction_account ta 
        ON tb.transaction_account_id = ta.transaction_account_id
    WHERE EXTRACT(YEAR FROM t.date) = EXTRACT(YEAR FROM CURRENT_DATE)
    GROUP BY ta.account_name
    ORDER BY ta.account_name;
END;
$$;
}}}

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

JOIN на табелите:
- J1 ← TA ⨝,,TA.transaction_account_id = TB.transaction_account_id,, TB
- J2 ← J1 ⨝,,TB.transaction_id = T.transaction_id,, T

Филтрирање по тековна година:
- F ← σ,,EXTRACT(YEAR FROM date) = EXTRACT(YEAR FROM CURRENT_DATE),,(J2)

Агрегација по квартали:
- G ← γ,,account_name; SUM(CASE WHEN EXTRACT(QUARTER FROM date) = 1 THEN spent_amount ELSE 0 END) → q1_spent, SUM(CASE WHEN EXTRACT(QUARTER FROM date) = 2 THEN spent_amount ELSE 0 END) → q2_spent, SUM(CASE WHEN EXTRACT(QUARTER FROM date) = 3 THEN spent_amount ELSE 0 END) → q3_spent, SUM(CASE WHEN EXTRACT(QUARTER FROM date) = 4 THEN spent_amount ELSE 0 END) → q4_spent,,(F)

Подредување по име на сметка:
- R_final ← τ,,account_name,,(G)