Отредактировано для доступа.Используйте UNION
для 2 подзапросов:
SELECT * FROM (
SELECT client.name, order.id AS orderid, order.product AS product,
"-" AS payid, "-" AS amount, order.date AS [date]
FROM client INNER JOIN [order] ON client.id = order.clientid
UNION
SELECT client.name, "-" AS orderid, "-" AS product,
pay.id AS payid, pay.amount AS amount, pay.date AS [date]
FROM client INNER JOIN [pay] ON client.id = pay.clientid
)
ORDER BY name, date
результат:
name orderid product payid amount date
a 1 px - - 2018-01-1
a - - 1 1000 2018-03-1
a 2 py - - 2018-05-1
b 3 px - - 2018-06-1
b - - 2 500 2018-09-1