Unique index behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a unique index on the column 'email' in the 'users' table.

SQL

CREATE UNIQUE INDEX [1] ON users(email);

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

Aunique_email_index

Busers_email_idx

Cidx_email_unique

Demail_idx

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-descriptive index name.

Omitting the UNIQUE keyword (not applicable here but common).

The index name idx_email_unique clearly indicates it is a unique index on the email column.

2fill in blank

medium

Complete the code to insert a new user with email '[e-mail]' into the 'users' table.

SQL

INSERT INTO users (name, email) VALUES ('Alice', [1]);

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A'[e-mail]'

B'[e-mail]'

C'[e-mail]'

D'[e-mail]'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different email than specified.

Forgetting to put quotes around the email.

The email '[e-mail]' matches the requirement for this insertion.

3fill in blank

hard

Fix the error in the query to select all users with unique emails.

SQL

SELECT * FROM users WHERE email [1] (SELECT email FROM users GROUP BY email HAVING COUNT(*) = 1);

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AIN

B=

CLIKE

DNOT IN

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which expects a single value.

Using 'LIKE' which is for pattern matching.

The IN operator is used to check if the email is in the list of emails that appear only once.

4fill in blank

hard

Fill both blanks to create a unique index on 'username' and 'email' columns in the 'accounts' table.

SQL

CREATE UNIQUE INDEX [1] ON accounts([2]);

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Aidx_username_email_unique

Busername

Cusername, email

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one column instead of both.

Using a non-descriptive index name.

The index name idx_username_email_unique is descriptive. The columns username, email are both included for uniqueness.

5fill in blank

hard

Fill all three blanks to select emails that appear more than once in the 'subscribers' table.

SQL

SELECT [1], COUNT([2]) FROM subscribers GROUP BY [3] HAVING COUNT(email) > 1;

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

Aemail

B*

Attempts:

3 left

💡 Hint

Common Mistakes

Counting the email column inside COUNT() instead of '*'.

Grouping by a different column than selected.

Select the email column, count all rows with *, and group by email to find duplicates.