Joins with grouping
HandoutReports: joins with grouping
The real power comes from combining what you have learned: join two tables, group the joined rows, and summarise each group with an aggregate.
A common report is "how much has each customer spent?" — join customers to their orders, group by customer, and SUM the totals.
One row per customer
SELECT customer.name,
COUNT(*) AS orders,
ROUND(SUM(orders.total), 2) AS spent
FROM customer
INNER JOIN orders ON customer.id = orders.customer_id
GROUP BY customer.id
ORDER BY customer.name;
Reading it in order: join the tables, group the rows by customer, then for each group count the orders and add up the totals.
Common mistakes
- Join first, then
GROUP BYto summarise the joined rows. - Qualify a column with its table name when both tables share it.
Joining tables
An inner join drops rows that have no match.
For each customer who has orders, show their name, their number of orders as orders, and their total spend (2 d.p.) as spent. Join, group by customer.id, and order by name.
Click Run to see the output here.
An INNER JOIN hid Ben and Dan, who have no orders. Use a LEFT JOIN to keep every customer, and COUNT(orders.id) so a customer with no orders shows 0. Order by name.
Click Run to see the output here.