Смотря в каком виде ожидается результат
SELECT * FROM `cell` WHERE `buy_cell` = 0 AND rate BETWEEN 0.01 AND 0.16 ORDER BY `rate` DESC
либо
SELECT *, GROUP_CONCAT(amount) FROM `cell`
WHERE `buy_cell` = 0 AND rate BETWEEN 0.01 AND 0.16
GROUP BY rate
ORDER BY `rate` DESC
