In MySQL, the PRIMARY KEY constraint is used to uniquely identify each record in a table. Primary key contains UNIQUE values only and the column defined as Primary key column can not be NULL. A table ...

CREATE TABLE emplyee ( id int NOT NULL, first_name varchar(45) NOT NULL, last_name varchar(45), country varchar(45), UNIQUE (id) ); We can also define UNIQUE constraint on multiple column, for this ...

ALTER TABLE `messages` ADD CONSTRAINT `user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`); ALTER TABLE `channel_details` ADD CONSTRAINT `user_id` FOREIGN KEY (`user_id`) REFERENCES ...