MySQL 8.0

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CREATE TABLE t (date date, price int);
INSERT INTO t VALUES
('2018-03-31', 115),
('2018-03-29', 114),
('2018-03-28', 113),
('2017-03-29', 117),
('2017-03-28', 118),
('2016-12-30', 143),
('2016-12-29', 140),
('2015-12-31', 110),
('2015-12-30', 111);
SELECT dates.dt, (
    SELECT price
    FROM t
    WHERE date <= dates.dt
    ORDER BY date DESC
    LIMIT 1
) AS price
FROM (
    SELECT '2018-03-31' AS dt UNION ALL
    SELECT '2017-03-31' UNION ALL
    SELECT '2016-12-31' UNION ALL
    SELECT '2015-12-31'
) AS dates


dt
price


2018-03-31
115

2017-03-31
117

2016-12-31
143

2015-12-31
110