Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table mytable (id varchar2(10), myfield varchar2(10));
insert into mytable (id, myfield) values ('id1', 'mydata1');
1 rows affected

insert into mytable (id, myfield) values ('id2', 'mydata2');
1 rows affected

select * from mytable where myfield <> ' ';

ID
MYFIELD

id1
mydata1

id2
mydata2

select * from mytable where myfield is not null;

ID
MYFIELD

id1
mydata1

id2
mydata2

select * from mytable where myfield <> null ;