COUNT(DISTINCT …)

Counts unique values instead of rows. Use it to count people or orders when a row can repeat.

FormulaCOUNT(*) counts rows · COUNT(DISTINCT x) counts different x

Try it · 30 seconds · from Atterna’s practice

SELECT COUNT(buyer_id)
FROM orders
WHERE status = 'paid';

Marketing asked for the number of customers. What does this query actually count?

Preparing the practice…

How we work it through

The query we run on Meridian, Atterna’s practice marketplace, and what it returns.

SELECT COUNT(*) AS paid_orders,
       COUNT(DISTINCT buyer_id) AS buyers
FROM meridian.orders
WHERE status = 'paid';

Result9,071 paid orders from 3,652 buyers

Using SUM(DISTINCT …) for money — two different orders can have the same amount.

ExampleMeridian: 9,071 paid orders come from 3,652 distinct buyers.

An AI assistant

“9,071 customers placed an order.”

What it didn’t know: It counted rows of orders and was not told a customer can order many times.
Ask back: “Is that orders or distinct buyers?”