Primary keys and uniqueness in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary key in a database table?

A primary key is a column or a set of columns that uniquely identifies each row in a table. It ensures no two rows have the same key value.

beginner

Can a primary key column contain NULL values?

No, a primary key column cannot contain NULL values because it must uniquely identify every row, and NULL means unknown or missing.

intermediate

What is the difference between a primary key and a unique constraint?

A primary key uniquely identifies rows and does not allow NULLs. A unique constraint also enforces uniqueness but allows NULL values depending on the database system.

beginner

Why is it important to have a primary key in a table?

Having a primary key helps to quickly find, update, or delete specific rows. It also helps maintain data integrity by preventing duplicate rows.

beginner

How do you define a primary key when creating a table in SQL?

You define a primary key using the PRIMARY KEY keyword in the CREATE TABLE statement, for example: CREATE TABLE Students (ID INT PRIMARY KEY, Name VARCHAR(50));

Which of the following is true about a primary key?

A It must be unique and cannot contain NULL values.

B It can contain duplicate values.

C It can contain multiple NULL values.

D It is optional for every table.

A primary key uniquely identifies each row and cannot have NULL values.

What happens if you try to insert a duplicate value into a primary key column?

A The database allows it without error.

B The database converts the duplicate to NULL.

C The database rejects the insert and shows an error.

D The duplicate overwrites the existing row.

Primary keys enforce uniqueness, so duplicates cause an error.

Which SQL keyword is used to enforce uniqueness on a column but allow NULL values?

A CHECK

B PRIMARY KEY

C FOREIGN KEY

D UNIQUE

The UNIQUE constraint enforces uniqueness but can allow NULLs.

Can a table have more than one primary key?

A Yes, multiple primary keys are allowed.

B No, only one primary key per table is allowed.

C Yes, but only if they are composite keys.

D No, primary keys are optional.

A table can have only one primary key, which can be a single or composite key.

What is a composite primary key?

A A primary key made of multiple columns combined.

B A primary key that allows NULL values.

C A primary key that is automatically generated.

D A primary key that references another table.

A composite primary key uses more than one column to uniquely identify rows.

Explain what a primary key is and why it is important in a database table.

Think about how you identify a person uniquely in real life.

Describe the difference between a primary key and a unique constraint.

Consider how strict each rule is about NULL values.