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Selecting data in SQL videos | SQL Queries Part 12 - Correlated Subqueries

Posted by Andrew Gould on 31 October 2012

SQL Queries Part 12 - Correlated Subqueries

A correlated subquery is a subquery which depends on an outer query for its values. If that doesn't make much sense, this video attempts to explain exactly what that means. You'll see how to build simple correlated subqueries, including why it's necessary to use table aliases, along with an explanation of what happens when you execute the query.

See our full range of SQL training resources, or test your knowledge of SQL with one of our SQL skills assessment tests.

This video has the following accompanying files:

File name	Type	Description
Create Movies Database with Genre Table.sql	SQL query
GenerateMoviesDatabase.sql	SQL query
GenerateUpdatedMoviesDatabase.sql	SQL query

Click to download a zipped copy of the above files.

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From:	Sanka
When:	07 Apr 21 at 10:17

Reply to Sanka

Hello Wise Owls,

I do have a question regarding the last case.

If the code in Line 14 is removed, will the output stays the same pls? Since the output is ORDERED BY y, we will always find the results rank by year, right?

Thanks in advance,

Sanka

From:	Andrew G
When:	12 Apr 21 at 07:39

Reply to Andrew G

Hi Sanka,

No, the results would be different if you remove line 14 from the last example. Line 14 is what makes this a correlated subquery. Without that line your results would return any film whose run time is longer than the average run time of every film in the table. With line 14 your results return each film whose run time is longer than the average run time of only those films made in the same year.

We aren't ranking results in this example, the ORDER BY clause was added simply to see the results more easily.

I hope that helps!

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