Oracle 18c

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CREATE TABLE mytable(
   ENTITY  VARCHAR(8) NOT NULL PRIMARY KEY
  ,VALUE_X INTEGER 
  ,VALUE_Y INTEGER 
);
begin
INSERT INTO mytable(ENTITY,VALUE_X,VALUE_Y) VALUES ('AAA',100,50);
INSERT INTO mytable(ENTITY,VALUE_X,VALUE_Y) VALUES ('BBB',0,20);
INSERT INTO mytable(ENTITY,VALUE_X,VALUE_Y) VALUES ('CCC',NULL,NULL);
INSERT INTO mytable(ENTITY,VALUE_X,VALUE_Y) VALUES ('DDD',100,100);
end;
/
1 rows affected

select t.*, x.*
from mytable t
outer apply (
	select col
	from (
		select 'X' col, t.value_x val from dual
		union all select 'Y', t.value_y from dual
	) x
	where x.val is not null
	order by x.val desc
	fetch first 1 row only
) x


ENTITY
VALUE_X
VALUE_Y
COL


AAA
100
50
X

BBB
0
20
Y

CCC
null
null
null

DDD
100
100
X