*****Newbie***** Pimary keys in db : Thanks folks! This from an online Khan tutorial on sql:

1) How do I know that student_id in students_grades is related to id in student?
2) Why is there no value for primary key in students?

Thanks a lot.

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CREATE TABLE students (id INTEGER PRIMARY KEY,
    first_name TEXT,
    last_name TEXT,
    email TEXT,
    phone TEXT,
    birthdate TEXT);

INSERT INTO students (first_name, last_name, email, phone, birthdate)
    VALUES ("Peter", "Rabbit", "peter@rabbit.com", "555-6666", "2002-06-24");
INSERT INTO students (first_name, last_name, email, phone, birthdate)
    VALUES ("Alice", "Wonderland", "alice@wonderland.com", "555-4444", "2002-07-04");
    
CREATE TABLE student_grades (id INTEGER PRIMARY KEY,
    student_id INTEGER,
    test TEXT,
    grade INTEGER);

INSERT INTO student_grades (student_id, test, grade)
    VALUES (1, "Nutrition", 95);
INSERT INTO student_grades (student_id, test, grade)
    VALUES (2, "Nutrition", 92);
INSERT INTO student_grades (student_id, test, grade)
    VALUES (1, "Chemistry", 85);
INSERT INTO student_grades (student_id, test, grade)
    VALUES (2, "Chemistry", 95);