SQL外部キー、外部キー制約を設定できません。

Question

CREATE TABLE students(
    student_id int(11) NOT NULL AUTO_INCREMENT, 
    first_name varchar(255) NOT NULL, 
    last_name varchar(255) NOT NULL, 
    advisor_id int(11) DEFAULT NULL, 
    PRIMARY KEY (student_id), 
    KEY advisor_id (advisor_id), 
    CONSTRAINT FOREIGN KEY (advisor_id) REFERENCES advisors (advisor_id) 
); 

Answer 1

CONSTRAINT (advisor_id) FOREIGN KEY REFERENCES advisors (advisor_id)