In both, or only in one: INTERSECT and EXCEPT

Two more ways to combine lists compare them instead of stacking them:

• INTERSECT keeps rows that appear in both results.
• EXCEPT keeps rows from the first result that aren't in the second.

-- Customers who have ordered online AND by phone
SELECT customer_id FROM orders WHERE channel = 'Online'
INTERSECT
SELECT customer_id FROM orders WHERE channel = 'Phone'

-- Customers who ordered in 2025 but not in 2026
SELECT customer_id FROM orders WHERE order_date >= '2025-01-01' AND order_date < '2026-01-01'
EXCEPT
SELECT customer_id FROM orders WHERE order_date >= '2026-01-01'

Both remove duplicates, like UNION does. Wrap one in brackets and SELECT COUNT(*) FROM (…) to count the result.

Other databases: EXCEPT is called MINUS in Oracle. MySQL only added INTERSECT and EXCEPT in version 8.0.31; on older versions, use EXISTS and NOT EXISTS.