Correlated sub-query
Sub-queries are used to retrieve information from another table, or query, that is separate to the main query.
A friend is working on a project looking at earthquake hazards around the world. She requires a table that lists all countries, their continent and the average magnitude earthquake by country. This query will need to access data from the Nations and Earthquakes tables.
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SELECT UNContinentRegion,
CountryName,
(SELECT ___(___) -- Add average magnitude
FROM Earthquakes e
-- Add country code reference
WHERE n.___ = e.Country) AS AverageMagnitude
FROM Nations n
ORDER BY UNContinentRegion DESC,
AverageMagnitude DESC;