MySQL 8.0

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CREATE TABLE `Inventar_actions` (
  `PartID` char(8) NOT NULL,
  `DateSeq` smallint(4) unsigned NOT NULL,
  `ActionType` enum('Bought','Sold') NOT NULL,
  `Units` bigint(20) unsigned NOT NULL,
  PRIMARY KEY (`PartID`,`DateSeq`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `Inventar_actions` (`PartID`, `DateSeq`, `ActionType`, `Units`) VALUES 
('0001', '1020', 'Bought', '100'),('0001', '1025', 'Sold', '20'),('0001', '1028', 'Sold', '30'),
('0001', '1031', 'Bought', '100'),('0001', '1035', 'Sold', '50');
CREATE TABLE `BusinessDates` (
  `DateSeq` int(10) unsigned NOT NULL,
  `BusinessDate` date NOT NULL,
  PRIMARY KEY (`DateSeq`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `BusinessDates` (`DateSeq`, `BusinessDate`) VALUES 
('1015', '2020-02-03'),('1016', '2020-02-04'),('1017', '2020-02-05'),('1018', '2020-02-06'),('1019', '2020-02-07'),
('1020', '2020-02-10'),('1021', '2020-02-11'),('1022', '2020-02-12'),('1023', '2020-02-13'),('1024', '2020-02-14'),
('1025', '2020-02-17'),('1026', '2020-02-18'),('1027', '2020-02-19'),('1028', '2020-02-20'),('1029', '2020-02-21'),
('1030', '2020-02-24'),('1031', '2020-02-25'),('1032', '2020-02-26'),('1033', '2020-02-27'),('1034', '2020-02-28'),
('1035', '2020-03-02'),('1036', '2020-03-03'),('1037', '2020-03-04'),('1038', '2020-03-05'),('1039', '2020-03-06')
;
SELECT 
   d.DateSeq
   , d.BusinessDate
   , a.`Units`
   , a.`ActionType`
      , 
   CASE 
     WHEN a.`ActionType` = 'Bought' THEN @inventory := @inventory + a.`Units`
   WHEN a.`ActionType` = 'Sold' THEN @inventory := @inventory - a.`Units`
   ELSE
    @inventory := @inventory
   END inventory
FROM (SELECT * FROM `BusinessDates` ORDER BY  BusinessDate ASC) AS d LEFT JOIN 
(SELECT DateSeq, ActionType, Units FROM `Inventar_actions` WHERE PartID = '0001') AS a USING (DateSeq)
,(SELECT @inventory := 0) b
WHERE d.DateSeq BETWEEN 1020 AND 1039
ORDER BY d.DateSeq;


DateSeq
BusinessDate
Units
ActionType
inventory


1020
2020-02-10
100
Bought
100

1021
2020-02-11
null
null
100

1022
2020-02-12
null
null
100

1023
2020-02-13
null
null
100

1024
2020-02-14
null
null
100

1025
2020-02-17
20
Sold
80

1026
2020-02-18
null
null
80

1027
2020-02-19
null
null
80

1028
2020-02-20
30
Sold
50

1029
2020-02-21
null
null
50

1030
2020-02-24
null
null
50

1031
2020-02-25
100
Bought
150

1032
2020-02-26
null
null
150

1033
2020-02-27
null
null
150

1034
2020-02-28
null
null
150

1035
2020-03-02
50
Sold
100

1036
2020-03-03
null
null
100

1037
2020-03-04
null
null
100

1038
2020-03-05
null
null
100

1039
2020-03-06
null
null
100

DateSeq	BusinessDate	Units	ActionType	inventory
1020	2020-02-10	100	Bought	100
1021	2020-02-11	null	null	100
1022	2020-02-12	null	null	100
1023	2020-02-13	null	null	100
1024	2020-02-14	null	null	100
1025	2020-02-17	20	Sold	80
1026	2020-02-18	null	null	80
1027	2020-02-19	null	null	80
1028	2020-02-20	30	Sold	50
1029	2020-02-21	null	null	50
1030	2020-02-24	null	null	50
1031	2020-02-25	100	Bought	150
1032	2020-02-26	null	null	150
1033	2020-02-27	null	null	150
1034	2020-02-28	null	null	150
1035	2020-03-02	50	Sold	100
1036	2020-03-03	null	null	100
1037	2020-03-04	null	null	100
1038	2020-03-05	null	null	100
1039	2020-03-06	null	null	100