PostgreSQL Aggregate Functions

Summary: in this tutorial, you will learn how to use the PostgreSQL aggregate functions such as AVG(), COUNT(), MIN(), MAX(), and SUM().

Introduction to PostgreSQL aggregate functions

Aggregate functions perform a calculation on a set of rows and return a single row. PostgreSQL provides all standard SQL’s aggregate functions as follows:

• AVG() – return the average value.
• COUNT() – return the number of values.
• MAX() – return the maximum value.
• MIN() – return the minimum value.
• SUM() – return the sum of all or distinct values.

In practice, you often use the aggregate functions with the GROUP BY clause in the SELECT statement:

SELECT column1, AGGREGATE_FUNCTION(column2)
FROM table1
GROUP BY column1;

In this syntax, the GROUP BY clause divides the result set into groups of rows and the aggregate function performs a calculation on each group e.g., maximum, minimum, average, etc.

PostgreSQL aggregate function examples

Let’s use the film table in the sample database for the demonstration.

[画像:Film table]

AVG() function examples

The following statement uses the AVG() function to calculate the average replacement cost of all films:

SELECT
 ROUND(AVG(replacement_cost), 2) avg_replacement_cost
FROM
 film;

The following is the result:

avg_replacement_cost
----------------------
 19.98
(1 row)

Noted that we use the ROUND() function to round the result to 2 decimal places.

To calculate the average replacement cost of the Drama films whose category id is 7, you use the following statement:

SELECT
 ROUND(
 AVG(replacement_cost),
 2
 ) avg_replacement_cost
FROM
 film
 INNER JOIN film_category USING(film_id)
 INNER JOIN category USING(category_id)
WHERE
 category_id = 7;

Here is the result:

avg_replacement_cost
----------------------
 21.09
(1 row)

COUNT() function examples

To get the number of films, you use the COUNT(*) function as follows:

SELECT
 COUNT(*)
FROM
 film;

Output:

count
-------
 1000
(1 row)

To get the number of drama films, you use the following statement:

SELECT
 COUNT(*) drama_films
FROM
 film
 INNER JOIN film_category USING(film_id)
 INNER JOIN category USING(category_id)
WHERE
 category_id = 7;

The result shows that there are 62 drama films:

drama_films
-------------
 62
(1 row)

MAX() function examples

The following statement returns the maximum replacement cost of films.

SELECT
 MAX(replacement_cost)
FROM
 film;

Output:

max
-------
 29.99
(1 row)

To get the films that have the maximum replacement cost, you use the following query:

SELECT
 film_id,
 title
FROM
 film
WHERE
 replacement_cost =(
 SELECT
 MAX(replacement_cost)
 FROM
 film
 )
ORDER BY
 title;

Output:

film_id | title
---------+-------------------------
 34 | Arabia Dogma
 52 | Ballroom Mockingbird
 81 | Blindness Gun
 85 | Bonnie Holocaust
 138 | Chariots Conspiracy
...

The subquery returned the maximum replacement cost which then was used by the outer query for retrieving the film’s information.

MIN() function examples

The following example uses the MIN() function to return the minimum replacement cost of films:

SELECT
 MIN(replacement_cost)
FROM
 film;

Output:

min
------
 9.99
(1 row)

To get the films that have the minimum replacement cost, you use the following query:

SELECT
 film_id,
 title
FROM
 film
WHERE
 replacement_cost =(
 SELECT
 MIN(replacement_cost)
 FROM
 film
 )
ORDER BY
 title;

Output:

film_id | title
---------+------------------------
 23 | Anaconda Confessions
 150 | Cider Desire
 182 | Control Anthem
 203 | Daisy Menagerie
...

SUM() function examples

The following statement uses the SUM() function to calculate the total length of films grouped by film’s rating:

SELECT
 rating,
 SUM(rental_duration)
FROM
 film
GROUP BY
 rating
ORDER BY
 rating;

The following picture illustrates the result:

rating | sum
--------+------
 G | 861
 PG | 986
 PG-13 | 1127
 R | 931
 NC-17 | 1080
(5 rows)

Summary

• Aggregate functions perform a calculation on a set of rows and return a single row.
• Use aggregate functions to summarize data.
• Use the AVG() function to calculate the average value in a set of values.
• Use the COUNT() function to perform a count.
• Use the SUM() function to calculate the total of values.
• Use the MIN() function to get the minimum value in a set of values.
• Use the MAX() function to get the maximum value in a set of values.