SQL DISTINCT: list unique values

SELECT DISTINCT country FROM customers lists each country once. With several columns, DISTINCT applies to the whole combination.

COUNT(DISTINCT customer_id) counts unique values, which is how you count customers rather than orders.

Worked example

Which sales channels have been used?

SELECT DISTINCT channel
FROM orders
ORDER BY channel;

Common mistakes

• Using DISTINCT to hide duplicates created by a wrong join instead of fixing the join.
• Expecting DISTINCT to apply to only one of several selected columns.