Primary keys and uniqueness in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a primary key on the 'id' column.

SQL

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

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APRIMARY KEY

BUNIQUE

CNOT NULL

DFOREIGN KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of PRIMARY KEY does not define the primary key.

Using FOREIGN KEY is incorrect here as it references another table.

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

2fill in blank

medium

Complete the code to add a unique constraint on the 'email' column.

SQL

ALTER TABLE users ADD CONSTRAINT unique_email [1] (email);

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APRIMARY KEY

BCHECK

CFOREIGN KEY

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using PRIMARY KEY here would be wrong because the table already has a primary key.

CHECK constraints are for conditions, not uniqueness.

The UNIQUE constraint ensures all values in the 'email' column are different.

3fill in blank

hard

Fix the error in the code to correctly define a composite primary key on 'order_id' and 'product_id'.

SQL

CREATE TABLE order_items (order_id INT, product_id INT, quantity INT, PRIMARY KEY ([1]));

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Aorder_id, product_id

Border_id

Cproduct_id

Dquantity

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one column instead of both for the composite key.

Including a non-key column like 'quantity' in the primary key.

A composite primary key uses multiple columns to uniquely identify a record.

4fill in blank

hard

Fill both blanks to create a table with a unique constraint on 'username' and a primary key on 'user_id'.

SQL

CREATE TABLE accounts (user_id INT [1], username VARCHAR(50) [2]);

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APRIMARY KEY

BNOT NULL

CUNIQUE

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing UNIQUE with PRIMARY KEY.

Not adding NOT NULL to primary key column (though PRIMARY KEY implies NOT NULL).

The 'user_id' column is the primary key, and 'username' must be unique.

5fill in blank

hard

Fill all three blanks to add a primary key and unique constraint using table-level constraints.

SQL

CREATE TABLE products (product_id INT, name VARCHAR(100), sku VARCHAR(50), CONSTRAINT [1] PRIMARY KEY ([2]), CONSTRAINT [3] UNIQUE (sku));

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Apk_products

Bproduct_id

Cuq_sku

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up constraint names or columns.

Using column names that are not keys for constraints.

Table-level constraints are named and specify primary key and unique constraints separately.