Keys (primary, candidate, foreign, super) in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Primary Key in a database?

A Primary Key is a unique identifier for each record in a database table. It ensures that no two rows have the same key value and cannot be null.

beginner

Define Candidate Key.

A Candidate Key is a column or a set of columns that can uniquely identify any record in a table. One of these candidate keys is chosen as the Primary Key.

beginner

What is a Foreign Key?

A Foreign Key is a column or set of columns in one table that refers to the Primary Key in another table. It creates a link between the two tables.

intermediate

Explain Super Key.

A Super Key is any set of columns that uniquely identifies a record in a table. It may contain extra columns beyond what is needed for uniqueness.

intermediate

How does a Candidate Key differ from a Super Key?

A Candidate Key is a minimal Super Key, meaning it has no unnecessary columns. A Super Key can have extra columns that are not needed for uniqueness.

Which key uniquely identifies each record in a table and cannot have null values?

A Primary Key

B Foreign Key

C Candidate Key

D Super Key

The Primary Key uniquely identifies each record and cannot be null.

What type of key is used to link two tables together?

A Candidate Key

B Primary Key

C Foreign Key

D Super Key

A Foreign Key in one table refers to the Primary Key in another, linking the tables.

Which key is a minimal set of columns that can uniquely identify a record?

A Foreign Key

B Candidate Key

C Super Key

D Primary Key

Candidate Keys are minimal unique identifiers; one is chosen as the Primary Key.

A Super Key may contain:

A Extra columns beyond what is needed for uniqueness

B Only one column

C Only foreign keys

D No columns

Super Keys can have extra columns that are not necessary for uniqueness.

Which key is chosen from candidate keys to uniquely identify records?

A Foreign Key

B Super Key

C Composite Key

D Primary Key

The Primary Key is selected from candidate keys to uniquely identify records.

Explain the differences between Primary Key, Candidate Key, Foreign Key, and Super Key.

Think about uniqueness and relationships between tables.

Why is it important to have a Primary Key in a database table?

Consider how databases organize and connect data.