MySQL 8.0

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create table YourTable (
 id int primary key not null auto_increment,
 `part number` varchar(30) not null,
 serial_number int,
 qty int not null default 0
);
insert into YourTable (`part number`, serial_number, qty) values
 ('A', 12345, 1)
,('A', 54321, 1)
,('A', 67890, 1)
,('B', NULL, 10)
,('B', NULL, 5)
,('C', NULL, 6)
,('C', NULL, 3)
;
SELECT *
FROM YourTable t
WHERE NOT EXISTS
(
   SELECT 1 
   FROM YourTable d
   WHERE d.`part number` = t.`part number`
     AND d.serial_number IS NULL
     AND t.serial_number IS NULL
     AND d.id < t.id
)
ORDER BY id;


id
part number
serial_number
qty


1
A
12345
1

2
A
54321
1

3
A
67890
1

4
B
null
10

6
C
null
6