MySQL : order by with null fields

COALESCE is a SQL function taking a list of parameters and returning the first not null. If every parameter is null, null is returned.

You can use COALESCE in an ORDER BY to create different sorting methods according to the fact that a field might be empty.

Sort with empty fields
SELECT
    *
FROM
    payment
ORDER BY
    COALESCE (date_refund, date_debit, date_create) DESC

-- OR

SELECT
    COALESCE (date_refund, date_debit, date_create) AS last_operation_date,
    p.*
FROM
    payment p
ORDER BY
    last_operation_date DESC

-- A field can be sorted by a specific value

SELECT
    COALESCE(
            date_refund,
            IF(payment_method = "bank card", NULL, date_debit),
            date_create
    ) AS last_operation_date,
    p.*
FROM
    payment p
ORDER BY
    last_operation_date DESC