==== Трендови на трошење според тагови за корисник
Сумирање на трошењата според тагови во последните 6 месеци за одреден корисник
{{{#!sql
CREATE OR REPLACE FUNCTION get_spending_trends_last_6_months_user(
    p_user_id INT
)
RETURNS TABLE (
    tag_name TEXT,
    month DATE,
    total_spent NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT 
        tg.tag_name, 
        DATE_TRUNC('month', t.date) AS month,
        COALESCE(SUM(tb.spent_amount), 0) AS total_spent
    FROM tag_assigned_to_transaction tat
    JOIN transaction t 
        ON tat.transaction_id = t.transaction_id
    JOIN transaction_breakdown tb 
        ON t.transaction_id = tb.transaction_id
    JOIN transaction_account ta 
        ON tb.transaction_account_id = ta.transaction_account_id
    JOIN tag tg 
        ON tat.tag_id = tg.tag_id
    WHERE ta.user_id = p_user_id
      AND t.date >= DATE_TRUNC('month', CURRENT_DATE - INTERVAL '6 MONTHS')
    GROUP BY tg.tag_name, month
    ORDER BY tg.tag_name, month;
END;
$$;
}}}

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

JOIN на сите табели:
- J1 ← TA ⨝,,TA.transaction_account_id = TB.transaction_account_id,, TB
- J2 ← J1 ⨝,,TB.transaction_id = T.transaction_id,, T
- J3 ← J2 ⨝,,T.transaction_id = TAT.transaction_id,, TAT
- J4 ← J3 ⨝,,TAT.tag_id = TG.tag_id,, TG

Филтрирање по корисник и последни 6 месеци:
- F ← σ,,user_id = p_user_id ∧ date >= DATE_TRUNC('month', CURRENT_DATE - INTERVAL '6 MONTHS'),,(J4)

Групирање по таг и месец:
- G ← γ,,tag_name, month = DATE_TRUNC('month', date); Σ(spent_amount) → total_spent,,(F)

Подредување по таг и месец:
- R_final ← τ,,tag_name, month,,(G)