Difference and Intersection: EXCEPT and INTERSECT

The List Comparison

Marketing asked: *"We have a list of all newsletter subscribers and a list of paying customers. How many subscribers are NOT customers? And how many are both?"*

The Quest: Set Operations

SQL has set operations that work like Venn diagrams.

**INTERSECT**: Returns rows that appear in BOTH queries.

**EXCEPT** (or `MINUS` in Oracle): Returns rows that appear in the first query but NOT in the second.

The Implementation: The Syntax

Who Subscribe But Never Paid?

SELECT email FROM newsletter_subscribers

EXCEPT

SELECT email FROM paying_customers;

Who Are Both Subscribers AND Customers?

SELECT email FROM newsletter_subscribers

INTERSECT

SELECT email FROM paying_customers;

The "Oops" Moment

I forgot that `EXCEPT` is directional. `A EXCEPT B` is different from `B EXCEPT A`.

**Pro Tip**: Think of `EXCEPT` as "Give me everything from the first set, but remove anything that's also in the second."

The Victory

Marketing discovered that 2,000 subscribers had never converted. They launched a targeted discount campaign and saw a 10% conversion uplift. Set operations are perfect for list analysis.

Your Task for Today

Use `EXCEPT` to find users who are in Table A but not in Table B. Then, use `INTERSECT` to find users who are in both.

*Day 46: Introduction to Subqueries.*