"How can I implement measures with one-to-many joins?" is a classic problem in dimensional modeling. Let's consider a simple example. Suppose we work with the dimensional model in the image above.

There are two measures in this model: sum(orders.total_cost) and sum(order_items.quantity). The "fact table" is orders, but we're using the term "fact table" loosely because orders has one-to-many relationships with order_items and returns. Suppose we want to answer the question:

• For each state…
• ...what is the total cost of all orders?
• ...and the total number of ordered items?

Put another way, we would like to construct the following pivot table:

Naively, we might use the following SQL query to answer this question:

select   
    state,   
    sum(total_cost),   
    sum(quantity)
from orders
join order_items using (order_id)
join customer using (customer_id)
group by 1

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However, this SQL query would produce a result that violates our intuition about the meaning of these measures. To understand why, suppose our tables are populated with the following data:

ORDERS

ORDER_ITEMS

CUSTOMERS

The result of our naive SQL query would be:

The sum(total_cost) for Alaska is $20, despite the fact that there was only one order in Alaska with a total cost of $10. This occurs because of the one-to-many join from orders to order_items; this single order was counted twice because it had two items in it.

Looker solves this problem using a clever trick called symmetric aggregates. (It is so clever they have patented it.) Looker would generate a query of the form:

select
    state, 
    sum(distinct hash(order_id) + total_cost) - sum(distinct hash(order_id)), 
    sum(quantity)
from orders
join order_items using (order_id)
join customer using (customer_id)
group by 1

The clever part is replacing sum(total_cost) with:

sum(distinct hash(order_id) + total_cost) - sum(distinct hash(order_id))

This funky-looking SQL ensures that each row of orders only contributes to the sum(total_cost) measure once. This approach works, but it has several disadvantages:

• It's complicated to write this SQL by hand.
• If you have multiple one-to-many joins, it can generate huge intermediate results.
• Distinct aggregates are slower than their simple counterparts.

If you're trying to accomplish the same thing with hand-written SQL, there is an alternative approach that solves these problems:

select 
    state, 
    sum(total_cost), 
    sum(quantity)	
from orders	
join (	    
    select order_id, sum(quantity) as quantity	   
    from order_items	    
    group by 1	
) using (order_id)	
join customers using (customer_id)	
group by 1

We have turned the one-to-many relationship into a one-to-one relationship by doing multiple levels of aggregation using a subquery. Subqueries have a bad reputation among analysts because older database systems execute them using inefficient recursive evaluation, but any modern data warehouse will generate an efficient plan from the above query.

As a bonus, we've avoided the use of more costly distinct aggregates, and there's no possibility of a "combinatorial explosion" producing huge intermediate results. To my knowledge, no BI tool currently uses this technique, but if you're writing SQL queries by hand, this is a useful trick.