PostgreSQL - SUM() Function

The SUM() function in PostgreSQL is used to calculate the sum of values in a numeric column. This article will guide you through the syntax, important considerations, and practical examples of using the SUM() function in PostgreSQL.

Syntax

SUM(column)

The following points need to be kept in mind while using the above function:

• NULL Values: The SUM() function ignores all NULL values in the column.
• DISTINCT Operator: When used with the 'DISTINCT' operator (e.g., 'SUM(DISTINCT column)'), it skips duplicate values.
• SELECT Clause: Using 'SUM()' with a 'SELECT' clause returns 'NULL' instead of zero if there are no matching rows.

PostgreSQL SUM() Function Examples

For example, we will be using the sample database (ie, dvdrental).

Example 1: Calculating Total Amount Paid by Each Customer

In this example, we will calculate the total amount paid by each customer using the SUM() function combined with the GROUP BY clause.

Query:

SELECT
    customer_id,
    SUM (amount) AS total
FROM
    payment
GROUP BY
    customer_id;

Output:

Explanation: The output will display the 'customer_id' and the total amount paid by each customer.

Example 2: Top 10 Customers Who Paid the Most

In this example we will query for the top 10 customers who paid the most as follows.

Query:

SELECT
    customer_id,
    SUM (amount) AS total
FROM
    payment
GROUP BY
    customer_id
ORDER BY total DESC
LIMIT 10;

Output:

Explanation: The output will display the 'customer_id' and the total amount paid by the top 10 customers.

Important Points About PostgreSQL SUM() Function

• The SUM() function automatically ignores NULL values in the calculation, which ensures that the sum is accurate based on non-null entries.
• When using the SUM() function on columns with decimal values, the result retains the precision of the column type, ensuring accurate summation of financial or other precise data.
• If the SUM() function is used with a SELECT clause and there are no matching rows, it returns NULL instead of zero.