SELECT round(sum(item_count*order_occurrences)/sum(order_occurrences),1) as meanFROM items_per_order

This SQL query is calculating the average number of items per order. Here's how it works:

1. item_count*order_occurrences: This part of the query multiplies the number of items in an order by the number of times that order occurred. It does this for each row in the items_per_order table.

2. sum(item_count*order_occurrences): This sums up the results of the above operation for all rows in the table. This gives us the total number of items ordered.

3. sum(order_occurrences): This sums up the number of times each order occurred. This gives us the total number of orders.

4. sum(item_count*order_occurrences)/sum(order_occurrences): This divides the total number of items ordered by the total number of orders. This gives us the average