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.
We often use the aggregate functions with the GROUP BY clause in the SELECT statement. In these cases, the GROUP BY clause divides the result set into groups of rows and the aggregate functions perform a calculation on each group e.g., maximum, minimum, average, etc.
You can use aggregate functions as expressions only in the following clauses:
-
SELECTclause. -
HAVINGclause.
PostgreSQL aggregate functions examples
Let’s use the film table in the sample database for the demonstration.
AVG() function examples
The following statement uses the AVG() function to calculate the average replacement cost of all films:
1 2 3 4 | SELECT ROUND( AVG( replacement_cost ), 2 ) avg_replacement_cost FROM film; |
The following is the result:
Noted that the ROUND() function was used 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:
1 2 3 4 5 6 7 8 | 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:
COUNT() function examples
To get the number of films, you use the COUNT(*) function as follows:
1 2 3 4 | SELECT COUNT(*) FROM film; |
Here is the output:
To get the number of drama films, you use the following statement:
1 2 3 4 5 6 7 8 | 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 showed that we have 62 drama films:
MAX() function examples
The following statement returns the maximum replacement cost of films.
1 2 3 4 | SELECT MAX(replacement_cost) FROM film; |
To get the films that have the maximum replacement cost, you use the following query:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | SELECT film_id, title FROM film WHERE replacement_cost =( SELECT MAX( replacement_cost ) FROM film ) ORDER BY title; |
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:
1 2 3 4 | SELECT MIN(replacement_cost) FROM film; |
To get the films which have the minimum replacement cost, you use the following query:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | SELECT film_id, title FROM film WHERE replacement_cost =( SELECT MIN( replacement_cost ) FROM film ) ORDER BY title; |
SUM() function examples
The following statement uses the SUM() function to calculate the total length of films grouped by film’s rating:
1 2 3 4 5 6 7 8 9 | SELECT rating, SUM( rental_duration ) FROM film GROUP BY rating ORDER BY rating; |
The following picture illustrates the result:
In this tutorial, you have learned about PostgreSQL aggregate functions and apply them to summarize data.