A key uniquely identifies a record in a database table. The primary key is the chosen candidate key that is less likely to change. A foreign key in one table references the primary key in another table to link the tables together in a relationship. Keys are represented visually with underlining for primary keys and asterisks for foreign keys.

2. Keys
A key is a field or set of fields whose values uniquely identify a record.
In this example, we could use SCN as a key because it uniquely identifies a
pupil record. No other pupil can have the same SCN (Scottish Candidate
Number).
No other fields can uniquely identify each pupil record

3. Candidate Key
Any field or set of fields which can be considered as a key is known as a
candidate key
In this example, both Employee Number and Email are candidate keys,
because both can uniquely identify the employee record in the table

4. Primary Key
The primary key is the candidate key which is chosen to be used
In the example above, it's possible that the company may change the structure
of staff email addresses. Given this, it would be better to use Employee
Number as a primary key as this is less likely to change.

5. Foreign Key
A foreign key is a column which is added to one table to create a relationship
with another table.
In this example we have two tables, teachers and departments in a school.
But there is no way to see which teacher works in which department.

6. Foreign Key
A foreign key is a column which is added to one table to create a relationship
with another table.
By adding the foreign key Department Code to the Teacher table, we can
create a relationship between the two tables.

7. Key Notation
We represent primary keys by underlining them.
We represent foreign keys with an asterisk.

8. Key Notation
We represent primary keys by underlining them.
We represent foreign keys with an asterisk.
young scot card (card_num, name, expiry_date, photo)
pupil (candidate_num, forename, surname, dob, card_num*)

9. Key Notation
We represent primary keys by underlining them.
We represent foreign keys with an asterisk.
pupil (candidate_num, forename, surname, dob, postcode)
exam (exam_id, subject, level, duration)
result (candidate_num*, exam_id*, grade)

10. Key Notation
We represent primary keys by underlining them.
We represent foreign keys with an asterisk.
movie (movie_id, name, certificate, rating)
cinema (cinema_id, city, telephone)
screening (cinema*, movie*, date, time)