How do I list the unique values in a column with SQL?

SELECT DISTINCT category FROM data;

`SELECT DISTINCT` removes duplicate rows from the result, so selecting just one column with `DISTINCT` gives you the list of unique values that column takes — useful for quickly checking what categories, statuses, or countries actually appear in a file before you decide how to group or filter it.

A table named data with columns: order_id (integer), customer (text), country (text), category (text), quantity (integer), unit_price (real), order_date (text, YYYY-MM-DD).