What is a Column Subquery?
Returns multiple values in one column (like a list).
Simple analogy: Getting a list of all customer IDs who ordered this month.
Basic Example with IN
Find students enrolled in any class:
SELECT name
FROM students
WHERE id IN (SELECT student_id FROM enrollments);How it works:
- Inner query gets list: 1, 2
- Outer query shows students with id IN (1, 2)
Real Example
Find customers who placed orders:
SELECT name
FROM customers
WHERE customer_id IN (SELECT customer_id FROM orders);Summary
Column subquery = Returns a list of values Use with IN, NOT IN