SQL Practice Problem #022 - Combine email lists without duplicates

Problem

Task

Return a single list of email addresses from customers and newsletter_signups without duplicates.

Schema

Table Schema

customers(id, email)
newsletter_signups(id, email)

Input

Sample Data

customers

id	email
1	[email protected]
2	[email protected]

newsletter_signups

id	email
10	[email protected]
11	[email protected]

Output

Expected Output

email
[email protected]
[email protected]
[email protected]

Answer

Check Your Solution

Show Answer and Explanation

Correct Answer

SELECT email
FROM customers
UNION
SELECT email
FROM newsletter_signups
ORDER BY email;

Explanation

UNION combines two result sets and removes duplicate rows. Because both SELECT statements return one email column, the sets are compatible. ORDER BY makes the final list stable.

Common Mistakes

Using UNION ALL when duplicates should be removed.
Selecting different numbers of columns on the two sides.
Expecting UNION to preserve source-table ordering.

Concepts

Related Concepts

UNION Set Operators Duplicate Handling Set Operator Duplicates