Cannot add foreign key constraint for mysql - mysql

Question

CREATE TABLE EMPLOYEE
(enum VARCHAR(5) NOT NULL,
first_name VARCHAR(30) NOT NULL,
Last_name VARCHAR(30) NOT NULL,
Salary decimal(8,2) NOT NULL,
CONSTRAINT EMPLOYEE_pkey PRIMARY KEY(enum));
CREATE TABLE LECTURER
(enum VARCHAR(5) NOT NULL,
lecturerNum CHAR(10) NOT NULL,
ranK VARCHAR(2) NOT NULL,
bioDate VARCHAR(200) NOT NULL,
CONSTRAINT pk_enum primary key(enum),
CONSTRAINT LECTURER_fkey FOREIGN KEY (enum) REFERENCES EMPLOYEE(enum),
CONSTRAINT lect_rank CHECK(rank in('L1'or'L2'or'L3')));
CREATE TABLE MANAGER
(enum VARCHAR(5) NOT NULL,
role VARCHAR(10) NOT NULL,
CONSTRAINT pk_enum primary key(enum),
CONSTRAINT MANAGER_fkey FOREIGN KEY(enum)
REFERENCES EMPLOYEE(enum) ON DELETE CASCADE);
CREATE TABLE SUBJECT
(subjectCode CHAR(10) NOT NULL,
SubjectName CHAR(30) NOT NULL,
credit decimal(3,3) NOT NULL,
taughtBy char(10) NOT NULL,
CONSTRAINT SUBJECT_PK PRIMARY KEY(subjectCode),
CONSTRAINT SUBJECT_fkey FOREIGN KEY(taughtBy)
REFERENCES LECTURER(lecturerNum) ON DELETE CASCADE,
CONSTRAINT CREDIT_CHECK CHECK(credit >0));
I did not find any error on my create table, why when I run it at the subject table it says:
ERROR 1215 (HY000): Cannot add foreign key constraint
Why..... can anyone help me with it?

Answer 1

LECTURER.lecturerNum does not have an index, which is required to be able to use it in a FK constraint:
CREATE TABLE LECTURER
(enum VARCHAR(5) NOT NULL,
lecturerNum CHAR(10) NOT NULL,
ranK VARCHAR(2) NOT NULL,
bioDate VARCHAR(200) NOT NULL,
KEY lecturerNum (lecturerNum),
CONSTRAINT pk_enum primary key(enum),
CONSTRAINT LECTURER_fkey FOREIGN KEY (enum) REFERENCES EMPLOYEE(enum),
CONSTRAINT lect_rank CHECK(rank in('L1'or'L2'or'L3')));