SQL’s union default order by clause?

I noticed when I am combining two tables via union that the order is not what I expected.

calculus_class

Kevin   Le      [e-mail]
Jackie  Chan    [e-mail]
Sam     Smit    [e-mail]

stats_class

Kay     Lam     [e-mail]
Jackie  Chan    [e-mail]
Pooja   Pri     [e-mail]

When I combine the two tables above

select * from calculus_class
union 
select * from stats_class 

I expect the results to be in order from top to bottom like this:

Kevin   Le      [e-mail]
Jackie  Chan    [e-mail]
Sam     Smit    [e-mail]
Kay     Lam     [e-mail]
Jackie  Chan    [e-mail]
Pooja   Pri     [e-mail]

This is the result I received using DBeaver PostgreSQL:

Kevin   Le      [e-mail]
Pooja   Pri     [e-mail]
Jackie  Chan    [e-mail]
Sam     Smit    [e-mail]
Kay     Lam     [e-mail]

Answer

Actually, you are using union which removes duplicates, but you don’t want duplicate removal. So just use union all:

select * from calculus_class
union all
select * from stats_class;

If you did want to order the results, you need to remember that SQL tables and result sets represent unordered sets. If you did want the results ordered by something, you could add an order by:

select * from calculus_class
union all
select * from stats_class
order by fname;

Here is a db<>fiddle.