Summary: this tutorial shows you how to use the PostgreSQL MAX function to get maximum value of a set.
Introduction to PostgreSQL MAX function
PostgreSQL MAX function is an aggregate function that you use to get the maximum value in a set. The MAXfunction is useful in many cases, for example to find the employees with the highest salary, most expensive products, etc.
The syntax of the MAX function is as follows:
1 | SELECT MAX(expression) |
You can use the MAX function not only in the SELECT clause but also in the WHERE and HAVING clauses. Let’s take a look at several examples of using the MAX function.
PostgreSQL MAX function examples
Let’s examine the paymenttable.
The following query gets the maximum amount paid by customers from the paymenttable:
1 2 | SELECT MAX(amount) FROM payment; |
PostgreSQL MAX function in subquery
To get other information together with the maximum payment, you use a subquery as follows:
1 2 3 4 5 6 7 8 9 10 11 | SELECT * FROM payment WHERE amount = ( SELECT MAX (amount) FROM payment ); |
First, the subquery gets the maximum payment and then the outer query selects all rows with the amount that is equal to the maximum payment returned from the subquery. The output of the query is as follows:
The following diagram illustrates the steps that PostgreSQL performs the query:
PostgreSQL MAX function with GROUP BY clause
You can also combine the MAXfunction with GROUP BY clause to get the maximum value per group. For example, the following query gets the largest payment paid by each customer for all customers.
1 2 3 4 5 6 7 | SELECT customer_id, MAX (amount) FROM payment GROUP BY customer_id; |
PostgreSQL MAX function with HAVING clause
If you use the MAXfunction in a HAVING clause, you can apply a filter for the whole group. For example, the following query selects only the biggest payment paid by each customer and the biggest payments are greater than 8.99.
1 2 3 4 5 6 7 8 | SELECT customer_id, MAX (amount) FROM payment GROUP BY customer_id HAVING MAX(amount) > 8.99 |
Finding largest values from two or more columns
Let’s create a new table named ranks for the demonstration. It has four columns: 1 column to store user id and other three columns to store rank from 1 to 3.
1 2 3 4 5 6 | CREATE TABLE IF NOT EXISTS ranks ( user_id INT PRIMARY KEY, rank_1 int4 NOT NULL, rank_2 int4 NOT NULL, rank_3 int4 NOT NULL ); |
We can insert sample data into the ranks table as follows:
1 2 3 4 5 | INSERT INTO ranks VALUES (1, 6, 3, 5), (2, 2, 8, 5), (3, 5, 9, 8); |
How do we get the largest rank per user:
To achieve this, you use GREATEST function instead of MAX function. The GREATEST function returns the greatest value from a list of values.
1 2 3 4 5 | SELECT user_id, GREATEST (rank_1, rank_2, rank_3) AS largest_rank FROM ranks; |
In this tutorial, you have learned how to use the PostgreSQL MAX function to find the maximum value of an expression. In addition, you learned how to use the MAX function with GROUP BY and HAVINGclauses.