==== Вкупно трошење според тагови
Вкупно трошење групирано според тагови за тековниот месец
{{{#!sql
CREATE OR REPLACE FUNCTION get_total_spent_by_tag_current_month()
RETURNS TABLE (
    tag_name TEXT,
    total_spent NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        tg.tag_name,
        COALESCE(SUM(tb.spent_amount), 0) AS total_spent
    FROM tag tg
    JOIN tag_assigned_to_transaction tat 
        ON tg.tag_id = tat.tag_id
    JOIN transaction t 
        ON tat.transaction_id = t.transaction_id
    JOIN transaction_breakdown tb 
        ON t.transaction_id = tb.transaction_id
    WHERE EXTRACT(MONTH FROM t.date) = EXTRACT(MONTH FROM CURRENT_DATE)
      AND EXTRACT(YEAR FROM t.date) = EXTRACT(YEAR FROM CURRENT_DATE)
    GROUP BY tg.tag_name
    ORDER BY total_spent DESC;
END;
$$;
}}}

==== Релациона алгебра
- TG(tag_id, tag_name)
- TAT(tag_id, transaction_id)
- T(transaction_id, date)
- TB(transaction_id, spent_amount)

JOIN на сите табели:
- J1 ← TG ⨝,,TG.tag_id = TAT.tag_id,, TAT
- J2 ← J1 ⨝,,TAT.transaction_id = T.transaction_id,, T
- J3 ← J2 ⨝,,T.transaction_id = TB.transaction_id,, TB

Филтрирање по тековен месец и година:
- F ← σ,,EXTRACT(MONTH FROM date) = EXTRACT(MONTH FROM CURRENT_DATE) ∧ EXTRACT(YEAR FROM date) = EXTRACT(YEAR FROM CURRENT_DATE),,(J3)

Групирање и агрегација по таг:
- G ← γ,,tag_name; SUM(spent_amount) → total_spent,,(F)

Подредување по вкупно потрошено:
- R_final ← τ,,total_spent DESC,,(G)