There are two issues in the provided code:

Incorrect Foreign Key Reference:
In the first CREATE TABLE statement, you have set the foreign key constraint on the "roll_no" column to reference the "age" column in the same "student" table. This does not seem to be the correct reference for a primary key. Typically, the primary key of the "student" table should be referenced by foreign keys in other tables.

To create a correct foreign key relationship, you should reference the primary key of another table (e.g., "grades" table) that matches the data type of the "roll_no" column. Assuming the primary key column in the "grades" table is named "student_id," you can correct the foreign key constraint as follows:

CREATE TABLE IF NOT EXISTS student (
  roll_no BIGINT,
  name VARCHAR(50),
  age INT,
  gender CHAR(1),
  hobbies VARCHAR(70),
  CONSTRAINT pk_roll_no PRIMARY KEY (roll_no)
);

CREATE TABLE IF NOT EXISTS grades (
  grade_id INT PRIMARY KEY,
  student_id BIGINT,
  -- Other grade-related columns here
  CONSTRAINT fk_student_id FOREIGN KEY (student_id) REFERENCES student(roll_no)
);

Missing Primary Key in "grades" Table:
In the ALTER TABLE statement, you are trying to add a foreign key constraint to the "grades" table, referencing the "student" table. However, it seems that the "grades" table itself does not have a primary key defined.

Before adding a foreign key constraint that references another table's primary key, you need to ensure that the referenced table has a primary key. In the corrected code above, I added a "grade_id" column as the primary key for the "grades" table.

By making these corrections, the foreign key constraint should work as expected. The "student_id" column in the "grades" table will reference the "roll_no" column in the "student" table. Remember to adjust the data types and column names to match your specific requirements if needed.

Doubt on Foreign key MySQL masterclass