How to create a foreign key in phpmyadmin

You can do it the old fashioned way... with an SQL statement that looks something like this

ALTER TABLE table_1_name
    ADD CONSTRAINT fk_foreign_key_name
    FOREIGN KEY (table_1_column_name)
    REFERENCES target_table(target_table_column_name)
    ON DELETE action_name
    ON UPDATE action_name;

For example: If you have books table with column created_by which refere to column id in users table:

ALTER TABLE books ADD CONSTRAINT books_FK_1 FOREIGN KEY (created_by) REFERENCES users(id) ON DELETE CASCADE ON UPDATE CASCADE;

Note: CASCADE mean if you delete from users table, related rows from books table will be deleted

This assumes the keys already exist in the relevant table

The key must be indexed to apply foreign key constraint. To do that follow the steps.

1. Open table structure. (2nd tab)
2. See the last column action where multiples action options are there. Click on Index, this will make the column indexed.
3. Open relation view and add foreign key constraint.

You will be able to assign DOCTOR_ID as foreign now.