UNIQUE constraints in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a UNIQUE constraint to the 'email' column in the 'users' table.

MySQL

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

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Aprimary_key

Bunique_email

Cindex_email

Dforeign_key

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'primary_key' as the constraint name instead of a unique constraint name.

Using 'index_email' which is an index, not a constraint.

The UNIQUE constraint requires a name, here 'unique_email' is a valid constraint name.

2fill in blank

medium

Complete the code to create a table 'products' with a UNIQUE constraint on the 'sku' column.

MySQL

CREATE TABLE products (id INT PRIMARY KEY, sku VARCHAR(50) [1] UNIQUE);

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ANOT NULL

BNULL

CDEFAULT

DAUTO_INCREMENT

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL which allows multiple NULLs and breaks uniqueness.

Using AUTO_INCREMENT on a VARCHAR column.

The 'sku' column should be NOT NULL to enforce uniqueness properly.

3fill in blank

hard

Fix the error in the code to add a UNIQUE constraint on 'username' in the 'accounts' table.

MySQL

ALTER TABLE accounts ADD UNIQUE [1] (username);

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Aunique_username

Busername

Cusername_unique

Dusername_key

Attempts:

3 left

💡 Hint

Common Mistakes

Using the column name 'username' as the constraint name causes confusion.

Using names that are not descriptive of the constraint purpose.

The constraint name should be a valid identifier; 'unique_username' is clear and valid.

4fill in blank

hard

Fill both blanks to create a UNIQUE constraint on 'email' and 'phone' columns in 'contacts' table.

MySQL

ALTER TABLE contacts ADD CONSTRAINT [1] UNIQUE ([2]);

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Aunique_contact

Bemail, phone

Cemail,phone

Dcontact_unique

Attempts:

3 left

💡 Hint

Common Mistakes

Using spaces after commas in column list causes syntax errors.

Using generic constraint names that don't describe the constraint.

The constraint name 'unique_contact' is valid, and columns must be listed without spaces after commas.

5fill in blank

hard

Fill all three blanks to create a table 'employees' with a UNIQUE constraint on 'email' and 'employee_id'.

MySQL

CREATE TABLE employees (id INT PRIMARY KEY, [1] VARCHAR(100) NOT NULL, [2] INT NOT NULL, CONSTRAINT [3] UNIQUE (email, employee_id));

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

Aemail

Bemployee_id

Cunique_emp

Demp_email

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up column names or missing NOT NULL on unique columns.

Using constraint names that are not descriptive.

'email' and 'employee_id' columns are defined, and 'unique_emp' is the constraint name.