MariaDB 10.2

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CREATE TABLE t (
  `Quantity` FLOAT,
  `BaseValue` INTEGER,
  `Id` INTEGER
);
INSERT INTO t
  (`Quantity`, `BaseValue`, `Id`)
VALUES
  ('-0.3', '1', '1'),
  ('-0.8', '2', '2'),
  ('0.5', '3', '3'),
  ('-0.2', '4', '4');
WITH RECURSIVE rcte AS (
    SELECT base.*, 0.0 + BaseValue AS y
    FROM t AS base
    WHERE Id = 1
    UNION ALL
    SELECT curr.*, GREATEST(prev.Quantity + prev.y, 0)
    FROM t    AS curr
    JOIN rcte AS prev ON curr.Id = prev.Id + 1
)
SELECT *
FROM rcte


Quantity
BaseValue
Id
y


-0.3
1
1
1.0

-0.8
2
2
0.7

0.5
3
3
0.0

-0.2
4
4
0.5