Can't add foreign keys because unique constraint needed

Question

I'm brand new to using Oracle SQLDeveloper and I'm working on a college project right now. I keep trying to add foreign key constraints to my tables(which already hold the foreign key as an attribute) so Im using ALTER like this:

alter table applies
add constraint e_number foreign key (e_number)
references student (e_number);

where e_number is the primary key in a table called student. The student table's e_number has the primary key constraint and also has an index that was auto-generated where it says UNIQUE under the UNIQUENESS column in the indexes tab. Whenever I try and create a foreign key for any of my tables I'm getting this same error everytime:

Error starting at line : 1 in command -
alter table applies
add constraint e_number foreign key (e_number)
references student (e_number)
Error report -
SQL Error: ORA-02264: name already used by an existing constraint
02264. 00000 -  "name already used by an existing constraint"
*Cause:    The specified constraint name has to be unique.
*Action:   Specify a unique constraint name for the constraint.

I'm a bit confused and have been reading about unique on several sites but still don't get it. When I call an ALTER I can either specify a FOREIGN key or specify a UNIQUE key. Am I supposed to ALTER unique and then ALTER foreign? What am I doing wrong?

Answer 1

It's because you already have a key named e_number. Try:

alter table applies
add constraint applies_student_e_number foreign key (e_number)
references student (e_number);