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.