sintaxe de chave estrangeira postgresql

CREATE TABLE students 
( 
  student_id SERIAL PRIMARY KEY,
  player_name TEXT
);

1. 
   Inline sem mencionar a coluna de destino:
   Testes

   CREATE TABLE tests 
   ( 
      subject_id SERIAL,
      subject_name text,
      highestStudent_id integer REFERENCES students
   );
   

2. 
   Em linha com a menção da coluna de destino:
   

   CREATE TABLE tests 
   ( 
      subject_id SERIAL,
      subject_name text,
      highestStudent_id integer REFERENCES students (student_id)
   );
   

3. 
   Fora de linha dentro da create table :
   Testes

   CREATE TABLE tests 
   ( 
     subject_id SERIAL,
     subject_name text,
     highestStudent_id integer, 
     constraint fk_tests_students
        foreign key (highestStudent_id) 
        REFERENCES students (student_id)
   );
   

4. 
   Como uma alter table separada demonstração:
   

   CREATE TABLE tests 
   ( 
     subject_id SERIAL,
     subject_name text,
     highestStudent_id integer
   );
   
   alter table tests 
       add constraint fk_tests_students
       foreign key (highestStudent_id) 
       REFERENCES students (student_id);