WITH u_c AS (
    SELECT
        a.username AS u,
        SUM(r.amount) AS c
    FROM accounts a
    JOIN readings r ON a.id = r.account_id
    GROUP BY a.username
)
SELECT
    u AS username,
    c AS consumption,
    CASE
        WHEN c <= 200 THEN CAST(CAST(c * (SELECT cost FROM tariffs WHERE name='A') AS DECIMAL(10,2)) AS CHAR)
        ELSE 'NULL'
    END AS `A(0-200)`,
    CASE
        WHEN c BETWEEN 201 AND 499 THEN CAST(CAST(c * (SELECT cost FROM tariffs WHERE name='B') AS DECIMAL(10,2)) AS CHAR)
        ELSE 'NULL'
    END AS `B(201-499)`,
    CASE
        WHEN c >= 500 THEN CAST(CAST(c * (SELECT cost FROM tariffs WHERE name='C') AS DECIMAL(10,2)) AS CHAR)
        ELSE 'NULL'
    END AS `C(500+)`
FROM u_c
ORDER BY u ASC;