lesson 09 Calculations & data quality · beginner

Unique values with DISTINCT

Return one copy of each distinct value.

the idea

DISTINCT removes duplicate result rows. It applies to the whole selected row: with two columns, a row is a duplicate only when both values repeat. Use it to answer “which values occur?”, not “how many rows are there?”.

Dataset: Retail

The tables the challenges use, with their first 6 rows. Under each column name: the kind of value it holds, and which table it links to.

worked example

Run these steps in order to see the idea before you write your own query. The step marked “the pattern” is what the challenges ask for. Running the example does not count toward progress.

1. 1

   See the same country many times. Run this. Country repeats across orders—those duplicates are what DISTINCT will remove.

   step-1.sql Run

   sql›

   SELECT order_id, country
   FROM orders
   LIMIT 15;

2. 2

   Keep each order type once. Run this. DISTINCT collapses the result to one row per different order_type value.the pattern

   example.sql Run

   sql›

   SELECT DISTINCT order_type
   FROM orders;

common mistake

SELECT DISTINCT country, region keeps one row per pair. It does not return each country only once if a country appears with two regions.