Diagnosing problems using anti join

Nice work on semi joins! The anti join is a related and powerful joining tool. It can be particularly useful for identifying whether an incorrect number of records appears in a join.

Say you are interested in identifying currencies of Oceanian countries. You have written the following INNER JOIN, which returns 15 records. Now, you want to ensure that all Oceanian countries from the countries table are included in this result. You'll do this in the first step.

SELECT c1.code, name, basic_unit AS currency
FROM countries AS c1
INNER JOIN currencies AS c2
ON c1.code = c2.code
WHERE c1.continent = 'Oceania';

If there are any Oceanian countries excluded in this INNER JOIN, you want to return the names of these countries. You'll write an anti join to this in the second step!