Вкупно трошење според тагови

Вкупно трошење групирано според тагови за тековниот месец

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)