Надминување на баланс на една сметка со трансакција во сегашно време

CREATE OR REPLACE FUNCTION get_current_account_overdrafts()
RETURNS TABLE (
    user_id INT,
    user_name TEXT,
    account_name TEXT,
    current_balance NUMERIC,
    transaction_id INT,
    transaction_name TEXT,
    transaction_amount NUMERIC,
    transaction_date TIMESTAMP
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        u.user_id,
        u.user_name,
        ta.account_name,
        ta.balance,
        t.transaction_id,
        t.transaction_name,
        tb.spent_amount,
        t.date
    FROM transaction_account ta
    JOIN "user" u ON ta.user_id = u.user_id
    JOIN transaction_breakdown tb ON ta.transaction_account_id = tb.transaction_account_id
    JOIN transaction t ON tb.transaction_id = t.transaction_id
    WHERE tb.spent_amount > ta.balance
      AND tb.spent_amount > 0
    ORDER BY t.date DESC;
END;
$$;

Релациона алгебра

• U(user_id, user_name)
• TA(transaction_account_id, user_id, account_name, balance)
• TB(transaction_id, transaction_account_id, spent_amount)
• T(transaction_id, transaction_name, date)

JOIN на сите табели:

• J1 ← TA ⨝_{TA.user_id = U.user_id} U
• J2 ← J1 ⨝_{TA.transaction_account_id = TB.transaction_account_id} TB
• J3 ← J2 ⨝_{TB.transaction_id = T.transaction_id} T

Филтрирање на трансакции што го надминуваат тековниот баланс:

• R ← σ_{spent_amount > balance ∧ spent_amount > 0}(J3)

Подредување хронолошки (од најнова кон најстара трансакција):

• R_final ← τ_{date DESC}(R)