Calculate Compressed Mean

Problem

Table: Orders

+-------------------+------+
| Column Name       | Type |
+-------------------+------+
| order_id          | int  |
| item_count        | int  |
| order_occurrences | int  |
+-------------------+------+ order_id is column of unique values for this table.
This table contains order_id, item_count, and order_occurrences.

Write a solution to calculate the average number of items per order, rounded to 2 decimal places.

Return the result table inany order .

The result format is in the following example.

Examples

Example 1:

    Input: 
    Orders table:
    +----------+------------+-------------------+
    | order_id | item_count | order_occurrences | 
    +----------+------------+-------------------+
    | 10       | 1          | 500               | 
    | 11       | 2          | 1000              |     
    | 12       | 3          | 800               |  
    | 13       | 4          | 1000              | 
    +----------+------------+-------------------+
    **Output**
    +-------------------------+
    | average_items_per_order | 
    +-------------------------+
    | 2.70                    |
    +-------------------------+
    **Explanation**
    The calculation is as follows:
     - Total items: (1 * 500) + (2 * 1000) + (3 * 800) + (4 * 1000) = 8900 
     - Total orders: 500 + 1000 + 800 + 1000 = 3300 
     - Therefore, the average items per order is 8900 / 3300 = 2.70

Solution

Method 1 – Weighted Average Using SUM and ROUND

Intuition

To find the average number of items per order, we need the total number of items (item_count × order_occurrences for each row) divided by the total number of orders (sum of order_occurrences). This is a weighted mean, and SQL's SUM and ROUND functions make it straightforward.

Approach

1. Use SUM to calculate the total items: sum of (item_count × order_occurrences).
2. Use SUM to calculate the total orders: sum of order_occurrences.
3. Divide total items by total orders to get the mean.
4. Use ROUND to round the result to 2 decimal places.
5. Return the result as average_items_per_order.

Code

MySQL

SELECT ROUND(SUM(item_count * order_occurrences) / SUM(order_occurrences), 2) AS average_items_per_order
FROM Orders;

Complexity

• ⏰ Time complexity: O(n) — n is the number of rows in Orders (single scan for SUM).
• 🧺 Space complexity: O(1) — Only accumulators are used.