如何使一张表的主键，同表的外键

Question

How to make primary key of one table, the foreign key of the same table?

I have table vendors with these attributes:

vendor_id,
company_name
rep_first
rep_last
referredby

Do I want to make vendor_id as the foreign key of that table?

Here is what I have tried:

create table vendors(
  vendor_id char(5) Not Null primary key,
  company_name varchar(255),
  rep_first varchar(255),
  rep_last varchar(255),
  referredby char(5)
);
constraint vendors_REF_FK
foreign key (referredby) references vendors(referredby)

Answer 1

Constraints is also part of the table and thus have to go along with table definition like

vendor_id char(5) Not Null primary key, 
company_name varchar(255), 
rep_first varchar(255), 
rep_last varchar(255), 
refferedby char(5),
constraint vendors_REF_FK foreign key (refferedby) references vendors(vendor_id ));

Answer 2

You may use ALTER TABLE as well

ALTER TABLE Vendors
ADD CONSTRAINT vendors_REF_FK FOREIGN KEY (refferedby) REFERENCES Vendors(vendor_id );