Lesson 23: UNION, INTERSECT & EXCEPT

Set operators combine the results of two queries — UNION merges, INTERSECT keeps only rows in both, EXCEPT keeps only rows in the first but not the second.

Real-time example

Build a mailing list combining active customers and newsletter-only subscribers, without duplicates.

SELECT email FROM customers WHERE active = true
UNION
SELECT email FROM newsletter_subscribers;

-- customers who bought but never subscribed to the newsletter:
SELECT email FROM customers
EXCEPT
SELECT email FROM newsletter_subscribers;

What's happening

UNION removes duplicate rows by default; UNION ALL keeps duplicates and is faster since it skips the dedup step.

Both queries in a set operation must return the same number of columns with compatible types.