单个主键的2个外键引用

Question

I have 3 tables: agreement, user, emails as follows and the id in user table is acting as a foreign key in email table(id(FK)) as well as agreement table id_two(FK).

I have referenced id(Foriegn Key) in emails table to id(Primary key) in user table using the below code successfully:

ALTER TABLE emails MODIFY COLUMN id INT NOT NULL,
 ADD CONSTRAINT id
 FOREIGN KEY(id)
 REFERENCES user(id);

But i am getting an error #1022 - Can't write; duplicate key in table '#sql-13f0_30e'

when i execute the below query to refer id_two of agreement table to id(PK) of user table.

ALTER TABLE agreement MODIFY COLUMN id_two INT NOT NULL,
 ADD CONSTRAINT id_two
 FOREIGN KEY(id_two)
 REFERENCES user(id);

</div>

Answer 1

That's most probably because you already have a constraint defined in agreement table named id_two. Change the name of the constraint and see

ALTER TABLE agreement MODIFY COLUMN id_two INT NOT NULL,
 ADD CONSTRAINT id_FK2
 FOREIGN KEY(id_two)
 REFERENCES user(id);