MySQL 5.6

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create table prices (`DATE` date, `ticker` varchar(10), `CLOSE` decimal(10,2))
insert into prices values
('2019-08-10', 'AAPL', 104.5),
('2019-08-12', 'AAPL', 104),
('2019-08-14', 'AAPL', 103.2),
('2019-08-15', 'AAPL', 105.6),
('2019-08-19', 'AAPL', 102.1),
('2019-08-21', 'AAPL', 100),
('2019-08-23', 'AAPL', 106.5),
('2019-08-24', 'AAPL', 109.7),
('2019-08-29', 'AAPL', 103.4),
('2019-08-31', 'AAPL', 104.1),
('2019-09-01', 'AAPL', 105.9),
('2019-09-04', 'AAPL', 102.2),
('2019-09-10', 'AAPL', 101.1),
('2019-09-12', 'AAPL', 104.4),
('2019-09-15', 'AAPL', 106),
('2019-09-20', 'AAPL', 107.5),
('2019-09-22', 'AAPL', 103.1),
('2019-09-25', 'AAPL', 102.2);
SELECT
    Y.day_20_High,
    tmax20.DATE AS day_20_High_date,
    Y.day_20_Low,
    tmin20.DATE AS day_20_Low_date,
    Y.day_20_Avg
FROM
    (
    SELECT
        MAX(CLOSE) AS day_20_High,
        MIN(CLOSE) AS day_20_Low,
        ROUND(AVG(CLOSE),2) AS day_20_Avg
    FROM
        `prices`
    WHERE
        DATE >= CURDATE() - INTERVAL 20 DAY AND ticker = "AAPL") Y
    LEFT JOIN prices tmax20 ON tmax20.close = Y.day_20_High
    LEFT JOIN prices tmin20 ON tmin20.close = Y.day_20_Low


day_20_High
day_20_High_date
day_20_Low
day_20_Low_date
day_20_Avg


107.50
2019-09-20
101.10
2019-09-10
104.05