Primary key

A column (or set of columns) whose value identifies exactly one row of a table and never repeats, such as order_id in an orders table.

FormulaCOUNT(*) = COUNT(DISTINCT key) — and it stays so

Try it · 30 seconds · from Atterna’s practice

order_id	buyer_id	seller_id	status
O009320	B02860	S032	paid
O009319	B02860	S007	paid
O009318	B02368	S008	paid

Which column can be the primary key of orders?

Preparing the practice…

How we work it through

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

SELECT COUNT(*),
       COUNT(DISTINCT order_id),
       COUNT(DISTINCT checkout_id)
FROM meridian.orders;

Result9,320 · 9,320 · 7,138 — order_id is a key, checkout_id only looks like one

Assuming a column is unique because it looks like an id. Check it: COUNT(*) against COUNT(DISTINCT id).

An AI assistant

“Each checkout_id is one order, so I joined payments on checkout_id.”

What it didn’t know: It assumed an id column is unique; one checkout splits into several seller orders.
Ask back: “Is checkout_id unique in orders? Compare COUNT(*) with COUNT(DISTINCT checkout_id).”