MySQL 8.0

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select version();


version()


8.0.30


CREATE TABLE MyNumbers(num INTEGER);

INSERT INTO MyNumbers VALUES
( 8   ),
( 8   ),
( 3   ),
( 3   ),
( 1   ),
( 4   ),
( 5   ),
( 6   );

Records: 8  Duplicates: 0  Warnings: 0


select  ifnull(num,null) as num 
from MyNumbers 
group by num
having count(num)=1
order by num desc
limit 1;


num


6


-- Make all values appear two times
INSERT INTO MyNumbers VALUES
( 1   ),
( 4   ),
( 5   ),
( 6   );

Records: 4  Duplicates: 0  Warnings: 0


-- This reproduces the question, no records returned
select  ifnull(num,null) as num 
from MyNumbers 
group by num
having count(num)=1
order by num desc
limit 1;


num


-- Because no value is having a count(num)=1, nothing is returned
select  count(num), ifnull(num,null) as num 
from MyNumbers 
group by num
-- having count(num)=1
order by num desc
-- limit 1
;


count(num)
num


2
8

2
6

2
5

2
4

2
3

2
1


-- What if we add `NULL` as a value in the table?
INSERT INTO MyNumbers VALUES
( NULL   );
select  count(num), ifnull(num,null) as num 
from MyNumbers 
group by num
-- having count(num)=1
order by num desc
-- limit 1
;


count(num)
num


2
8

2
6

2
5

2
4

2
3

2
1

0
null


select  count(num), count(*), ifnull(num,null) as num 
from MyNumbers 
group by num
-- having count(num)=1
order by num desc
-- limit 1
;


count(num)
count(*)
num


2
2
8

2
2
6

2
2
5

2
2
4

2
2
3

2
2
1

0
1
null


-- solution:
select  ifnull(num,null) as num 
from MyNumbers 
group by num
having count(*)=1
order by num desc
limit 1
;


num


null