How to Calculate the Sum of a Column for Unique Rows in a Grouped SQL Query?

MYSQL: Using sum() for Unique Rows

In SQL queries, the sum() function is typically used to calculate the sum of values across multiple rows. However, when working with grouped data, it is essential to consider duplicate rows. In this specific question, the user aims to calculate the sum of a column conversions.value for each distinct row in the conversions table.

The query provided initially encounters an issue where sum(conversions.value) incorrectly counts the value for each row multiple times due to the GROUP BY clause. To address this, the user must ensure that the sum is calculated only for unique rows.

The solution lies in utilizing the concept of a Cartesian product, which combines elements of multiple sets. In this case, the sets are the unique click and conversion events. The count(DISTINCT conversions.id) function effectively removes duplicate conversion rows, providing the count of unique conversion events.

To calculate the correct sum, the user can multiply sum(conversions.value) by the count of distinct conversions and divide it by the total count of rows (count(*)). This operation ensures that the sum is computed only for unique conversion events associated with each link.

The modified query recommended in the solution is:

<code class="sql">SELECT links.id, 
       count(DISTINCT stats.id) as clicks, 
       count(DISTINCT conversions.id) as conversions, 
       sum(conversions.value)*count(DISTINCT conversions.id)/count(*) as conversion_value 
FROM links 
LEFT OUTER JOIN stats ON links.id = stats.parent_id 
LEFT OUTER JOIN conversions ON links.id = conversions.link_id 
GROUP BY links.id 
ORDER BY links.created desc;</code>

The above is the detailed content of How to Calculate the Sum of a Column for Unique Rows in a Grouped SQL Query?. For more information, please follow other related articles on the PHP Chinese website!