Keys

Primary Key

The Attribute helps to identify a row in an entity uniquely.

It cannot be empty and cannot have duplicates.

student id	name	join date
101	Rachel Green	2020-05-01
201	Joey Tribianni	1998-07-05
301	Monica Geller	1999-12-14
401	Cosmo Kramer	2001-06-05

Based on the above data, which attribute can be PK?

any column (as its just four rows)

If we extend the dataset to 10000 rows, which can be PK?

yes student id

Composite Key

A Primary key that consists of two or more attributes is known as a **** composite key.

student id	course id	name
101	C1	Rachel Green
101	C2	Rachel Green
201	C2	Monica Geller
201	C3	Cosmo Kramer

Do you know how to find the unique row?

The combination of StudentID + CourseID makes it unique.

Unique Key

Unique keys are similar to Primary Key, except it can allow one NULL.