NULL and missing values

The idea

NULL represents an unknown or missing value. Test it with IS NULL, not = NULL. Read the query line by line and identify the input table, the transformation, and the shape of the result before running it.

SELECT name FROM customers WHERE country IS NULL;

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: Use WHERE country IS NULL. The sample data may return zero rows. Close this page section mentally and explain the query from memory before proceeding.

Run your own query

Find customers whose country has not been recorded.

Use WHERE country IS NULL. The sample data may return zero rows.

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.