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.
Questo esercizio fa parte del corso
Improving Query Performance in SQL Server
Esercizio pratico interattivo
Prova a risolvere questo esercizio completando il codice di esempio.
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;