How can you create a foreign key and link 2 tables?

Answer 1

are the "user_id" and "comment_id" the primary keys for each tables? because I am not getting it
because when I enter this command it says that the foreign key constraint is formed incorrectly

Answer 2

Sorry, I did not realize that my initial answer was wrong.

I made a working example and tested it on MySQL server version 5.7.20:

CREATE DATABASE db_test;

USE db_test;

CREATE TABLE tbl_user (
-- Primary key
id int PRIMARY KEY NOT NULL AUTO_INCREMENT,
-- Some other attributes
firstname varchar(100) NOT NULL,
lastname varchar(100) NOT NULL
);

CREATE TABLE tbl_comment (
-- Primary key
id int PRIMARY KEY NOT NULL AUTO_INCREMENT,
-- Foreign key
user_id int NOT NULL,
-- Some other attributes
headline varchar(50) NOT NULL,
content varchar(255) NOT NULL
);

-- Set the foreign key constraint
ALTER TABLE tbl_comment
ADD CONSTRAINT fk_user_comment FOREIGN KEY (user_id) REFERENCES tbl_user (id);

I hope this example is more clear.

Answer 3

i typed this exactly but unfortunately it didn't work again. thank u tho