Summaries answer most everyday questions: how many, how much, on average, for each something. This set uses the support tickets, employees, purchase_orders and orders.
Watch for the cases that change an answer without an error:
AVG and COUNT(column) skip NULLs. Many tickets have no satisfaction score.GROUP BY treats NULL as one group of its own, such as orders with no coupon.- A condition on a total belongs in
HAVING; a condition on single rows belongs in WHERE.
Round averages to two decimal places when the task says so.