Sub-query vs INNER JOIN
Often the results from a correlated sub-query can be replicated using an INNER JOIN
. Depending on what your requirements are, using an INNER JOIN
may be more efficient because it only makes one pass through the data whereas the correlated sub-query must execute for each row in the outer query.
You want to find out the 2017 population of the biggest city for every country in the world. You can get this information from the Earthquakes
database with the Nations
table as the outer query and Cities
table in the sub-query.
You will first create this query as a correlated sub-query then rewrite it using an INNER JOIN
.
This exercise is part of the course
Improving Query Performance in SQL Server
Hands-on interactive exercise
Have a go at this exercise by completing this sample code.
SELECT
n.CountryName,
(SELECT MAX(c.___) -- Add 2017 population column
FROM Cities AS c
-- Outer query country code column
WHERE c.CountryCode = n.___) AS BiggestCity
FROM ___ AS n; -- Outer query table