DISTINCT and deduplication

The idea

DISTINCT returns unique combinations of selected columns. Read the query line by line and identify the input table, the transformation, and the shape of the result before running it.

SELECT DISTINCT country FROM customers ORDER BY country;

Explain it without looking

In your own words, explain the main SQL feature in today’s lesson and why it answers the question.

Recall prompt: SELECT DISTINCT country, then ORDER BY country. Close this page section mentally and explain the query from memory before proceeding.

Run your own query

List each country represented in customers exactly once.

SELECT DISTINCT country, then ORDER BY country.

Close the loop

• Run the query and inspect its output.
• Change one part of the query and predict what will happen.
• Return later and rewrite the query from memory.