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Subqueries and Aggregate Functions

Written by Andrew Gould

In this tutorial

A Reminder of the Aggregate Functions
Finding the Highest or Lowest Value in a Column
Finding Records Above or Below Average
Adding More Criteria

Subqueries are often used with aggregate functions such as MAX - this part of the series will also show a couple of the other useful functions that you can use.

A Reminder of the Aggregate Functions

You might find this tutorial on grouping data useful as a reminder of which aggregate functions are available in SQL.

Finding the Highest or Lowest Value in a Column

We've already seen how to use the MAX function to match records against the largest value in a column. You can also use the MIN function to find the smallest value in a field. The query below would find the film or films with the shortest running time:

SELECT

FilmName

,FilmRunTimeMinutes

FROM

tblFilm

WHERE

FilmRunTimeMinutes =

(SELECT

MIN(FilmRunTimeMinutes)

FROM

tblFilm)

The results of this query are shown below:

Short duration, long title!

Finding Records Above or Below Average

You can use the AVG function in a subquery to return the average of a column of numbers. You can then combine this with the greater-than and less-than operators to find records that are above or below average. The query below shows all of the films whose running time is longer than the average running time of all the films in the table:

SELECT

FilmName

,FilmRunTimeMinutes

FROM

tblFilm

WHERE

FilmRunTimeMinutes >

(SELECT

AVG(FilmRunTimeMinutes)

FROM

tblFilm)

Adding More Criteria

You can add further criteria to the WHERE clause of both the inner and outer queries to make your queries even more specific. The example below looks for all films which have been released since the year 2000, and whose budget is greater than the highest budget of any film released before the year 2000.

SELECT

FilmName

,FilmBudgetDollars

FROM

tblFilm

WHERE

FilmReleaseDate >= '2000-01-01'

AND

FilmBudgetDollars >

(SELECT

MAX(FilmBudgetDollars)

FROM

tblFilm

WHERE

FilmReleaseDate < '2000-01-01')

The results of the query are shown below:

Three films in our database cost more to make than the most expensive film made before the year 2000.

Technically, we don't need the criterion which asks for films released on or after January 1st 2000. The only films that could be more expensive than the most expensive film made prior to that date must have been made after that date! I've included it here just to show how to use multiple criteria.

Now that you've seen how to use basic criteria with subqueries it's time to take a look at some of the more exotic comparisons using keywords such as ALL, ANY and IN.

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Creating a Simple Subquery

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Using ALL, ANY, SOME and IN with Subqueries

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CTEs and derived tables

You can learn more about this topic on the following Wise Owl courses:

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Online tutorials

Tutorials on SQL

Subqueries

Subqueries and Aggregate Functions

A Reminder of the Aggregate Functions

Finding the Highest or Lowest Value in a Column

Finding Records Above or Below Average

Adding More Criteria

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