transfer_out.pls

CREATE OR REPLACE FUNCTION
    transfer_out(source VARCHAR, amount INT) RETURNS void AS $$
DECLARE
    source_balance_after int;
BEGIN
    UPDATE balances SET balance = balance - amount WHERE id = source;
    SELECT balance FROM balances WHERE id = source INTO source_balance_after;
    IF source_balance_after < 0 THEN
        RAISE EXCEPTION 'Account % does not have enough money', source;
    END IF;
END
$$
LANGUAGE 'plpgsql' ;