Column constraints (NOT NULL, UNIQUE, CHECK) in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the email column not accept empty values.

PostgreSQL

CREATE TABLE users (id SERIAL PRIMARY KEY, email VARCHAR(255) [1]);

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ANOT NULL

BUNIQUE

CCHECK

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of NOT NULL, which allows NULL values but prevents duplicates.

Using CHECK without a proper condition.

The NOT NULL constraint ensures that the email column cannot have empty (NULL) values.

2fill in blank

medium

Complete the code to ensure the username column has unique values.

PostgreSQL

CREATE TABLE accounts (id SERIAL PRIMARY KEY, username VARCHAR(50) [1]);

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ANOT NULL

BCHECK

CUNIQUE

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOT NULL only, which does not prevent duplicates.

Using CHECK without a condition.

The UNIQUE constraint ensures that no two rows have the same username value.

3fill in blank

hard

Fix the error in the age column to allow only ages 18 or older.

PostgreSQL

CREATE TABLE members (id SERIAL PRIMARY KEY, age INT [1] (age >= 18));

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ACHECK

BUNIQUE

CNOT NULL

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOT NULL which only prevents empty values.

Using UNIQUE which prevents duplicates but not value ranges.

The CHECK constraint with condition age >= 18 restricts values to 18 or older.

4fill in blank

hard

Fill both blanks to create a price column that cannot be empty and must be positive.

PostgreSQL

CREATE TABLE products (id SERIAL PRIMARY KEY, price NUMERIC [1] [2] (price > 0));

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ANOT NULL

BUNIQUE

CCHECK

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of NOT NULL.

Omitting the CHECK condition for positive values.

NOT NULL ensures price is not empty, and CHECK (price > 0) ensures it is positive.

5fill in blank

hard

Fill all three blanks to create a user_id column that is unique, not empty, and must be greater than zero.

PostgreSQL

CREATE TABLE orders (order_id SERIAL PRIMARY KEY, user_id INT [1] [2] [3] (user_id > 0));

Drag options to blanks, or click blank then click option'

ANOT NULL

BUNIQUE

CCHECK

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing order of constraints incorrectly.

Forgetting NOT NULL or CHECK constraints.

UNIQUE ensures no duplicate user_id, NOT NULL prevents empty values, and CHECK (user_id > 0) ensures positive values.