Below are some example queries/use cases used in day to day data engineering tasks. 

suppose, you are given three tables customers, orders and orders_payment. You are asked to find the city wise ordered amount from these tables.

To find out the city wise total ordered amount from above three tables, we need to use join and aggregate functions.

select cust.customer_city, sum(paymnt.payment_value) as total_amount from  `hardy-position-352014.ecommerce.customers` cust  left join `hardy-position-352014.ecommerce.orders` ord on cust.customer_id = ord.customer_id left join `hardy-position-352014.ecommerce.orders-payments` paymnt on ord.order_id =  paymnt.order_id group by cust.customer_city