WITH RankedTransactions AS (
    SELECT
        customer_id,
        amount,
        dt,
        ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY dt ASC) AS rn
    FROM
        transactions
)
SELECT
    c.iban,
    GROUP_CONCAT(
        CASE
            WHEN t.rn = 1 THEN t.amount
            WHEN t.amount < 0 THEN CONCAT('- ', ABS(t.amount))
            ELSE CONCAT('+ ', t.amount)
        END
        ORDER BY t.dt ASC
        SEPARATOR ' '
    ) AS transactions,
    SUM(t.amount) AS total
FROM
    customers c
JOIN
    RankedTransactions t ON c.id = t.customer_id
GROUP BY
    c.iban
ORDER BY
    total DESC;