-- You're trying to find the mean number of items per order on Alibaba, rounded to 1 decimal place using tables which includes information on the count of items in each order (item_count table) and the corresponding number of orders for each item count (order_occurrences table). SELECT ROUND(SUM(item_count::decimal * order_occurrences) / SUM(order_occurrences), 1) as mean FROM items_per_order -- other approaches: -- 1. ROUND((SUM(order_occurrences * item_count) / SUM(order_occurrences))::numeric, 1) AS mean -- 2. ROUND(CAST(SUM(item_count*order_occurrences) / SUM(order_occurrences) AS DECIMAL),1) AS mean -- my approach: -- mean items per order -- round to 1 SELECT ROUND(SUM(item_count * order_occurrences) / SUM(order_occurrences), 1) as mean FROM items_per_order -- remarks : both item_count and order_occurrences are of integer type by default, which means that division will return an integer result. To ensure that the output is rounded to 1 decimal place, we can cast either column to a decimal type.