SELECT
kpo.date,
kpo.`сумма приход`,
kup.`сумма уход`,
(kup.`сумма уход`)/kpo.`сумма приход` AS Соотношение


FROM (SELECT
SUM(kp.amount) AS `сумма приход`,
kp.date

FROM kassa_prihod AS kp
WHERE kp.order_id IS NOT NULL
-- AND kassa_id <> 99
Group by date_format(kp.date, "%Y-%m")) AS kpo

LEFT JOIN (SELECT
SUM(ku.amount) AS `сумма уход`,
ku.date

FROM kassa_uhod AS ku
WHERE ku.kod=10100
Group by date_format(ku.date, "%Y-%m")) AS kup ON kup.date=kpo.date