Why constraints matter in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a table with a primary key constraint.

SQL

CREATE TABLE employees (id INT [1], name VARCHAR(100));

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ACHECK

BFOREIGN KEY

CUNIQUE

DPRIMARY KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOREIGN KEY instead of PRIMARY KEY

Forgetting to add any constraint

The PRIMARY KEY constraint uniquely identifies each record in the table.

2fill in blank

medium

Complete the code to add a NOT NULL constraint to the email column.

SQL

CREATE TABLE users (user_id INT PRIMARY KEY, email VARCHAR(255) [1]);

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AUNIQUE

BCHECK

CNOT NULL

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of NOT NULL

Leaving the column without any constraint

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

3fill in blank

hard

Fix the error in the constraint syntax to ensure age is always 18 or older.

SQL

CREATE TABLE members (member_id INT PRIMARY KEY, age INT CHECK (age [1] 18));

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A>=

B<

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator which allows younger ages

Using equality operator which only allows exactly 18

The CHECK constraint with >= 18 ensures age is 18 or older.

4fill in blank

hard

Fill both blanks to create a foreign key constraint linking orders to customers.

SQL

CREATE TABLE orders (order_id INT PRIMARY KEY, customer_id INT, CONSTRAINT fk_customer FOREIGN KEY ([1]) REFERENCES [2](id));

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Acustomer_id

Borders

Ccustomers

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the wrong table

Using the wrong column name for the foreign key

The foreign key customer_id in orders references the id in customers table.

5fill in blank

hard

Fill all three blanks to create a table with a unique constraint and a default value.

SQL

CREATE TABLE products (product_id INT PRIMARY KEY, sku VARCHAR(50) [1], stock INT [2] 0, CONSTRAINT unique_sku [3] (sku));

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AUNIQUE

BDEFAULT

DCHECK

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing UNIQUE and CHECK constraints

Forgetting to set a default value for stock

The sku column has a UNIQUE constraint to avoid duplicates, stock has a DEFAULT value of 0, and the constraint named unique_sku enforces uniqueness on sku.