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.
Cet exercice fait partie du cours
Improving Query Performance in SQL Server
Exercice interactif pratique
Essayez cet exercice en complétant cet exemple de code.
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;