MariaDB 10.3

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create table Transactions (code varchar(10), qty int, date datetime primary key);
insert into Transactions values('123', 100, '2018-01-01 00:00:00');
insert into Transactions values('123', 100, '20180102');
insert into Transactions values('123', 100, '20180103');
insert into Transactions values('123', 100, '20180104');
insert into Transactions values('123', 100, '20180105');
SELECT date,
       qty, 
       @acm := @acm + qty as stock
FROM
    Transactions t,
    (SELECT @acm := 0) x


date
qty
stock


2018-01-01 00:00:00
100
100

2018-01-02 00:00:00
100
200

2018-01-03 00:00:00
100
300

2018-01-04 00:00:00
100
400

2018-01-05 00:00:00
100
500

date	qty	stock
2018-01-01 00:00:00	100	100
2018-01-02 00:00:00	100	200
2018-01-03 00:00:00	100	300
2018-01-04 00:00:00	100	400
2018-01-05 00:00:00	100	500