MySQL 8.0

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CREATE TABLE mytable(
   ID   INTEGER  NOT NULL 
  ,Item VARCHAR(30)  NOT NULL
);

INSERT INTO mytable(ID,Item) VALUES (1,'Part 1');
INSERT INTO mytable(ID,Item) VALUES (1,'Part 2');
INSERT INTO mytable(ID,Item) VALUES (1,'Part 3');
INSERT INTO mytable(ID,Item) VALUES (1,'Part 4');
INSERT INTO mytable(ID,Item) VALUES (2,'Part 1');
INSERT INTO mytable(ID,Item) VALUES (2,'Part 2');
INSERT INTO mytable(ID,Item) VALUES (2,'Part 4');
INSERT INTO mytable(ID,Item) VALUES (2,'Part 5');
INSERT INTO mytable(ID,Item) VALUES (3,'Part 2');
INSERT INTO mytable(ID,Item) VALUES (3,'Part 4');
INSERT INTO mytable(ID,Item) VALUES (3,'Part 5');
select t1.id, t2.item
from (select distinct id from mytable) t1
cross join (select distinct item from mytable) t2
left join mytable t on t.id = t1.id and t.item = t2.item
where t.id is null
order by t1.id, t2.item


id
item


1
Part 5

2
Part 3

3
Part 1

3
Part 3


    select t1.id, t2.item
    from (select distinct id from mytable) t1
    cross join (select distinct item from mytable) t2
    where not exists (
		select 1
		from mytable t
		where t.id = t1.id and t.item = t2.item
	)


id
item


3
Part 1

3
Part 3

2
Part 3

1
Part 5