Ordering
Eliminating duplicate results
Aggregation
Grouping
19. Group by a few columns
HAVING: filtering and ordering groups
Let's practice

## Instruction

Nice work.

There's one more thing about GROUP BY that we want to discuss. Sometimes we want to group the rows by more than one column. Let's imagine we have a few customers who place tons of orders every day, so we would like to know the daily sum of their orders.

SELECT
customer_id,
order_date,
sum(total_sum)
FROM orders
GROUP BY customer_id, order_date;


As you can see, we group by two columns: customer_id and order_date. We select these columns alongside with the function sum(total_sum).

Remember: in such queries each column in the SELECT part must either be used later for grouping or it must be used with one of the functions.

It makes no sense to select any other column. For example, each order on the very same day by the very same customer can have a different shipping date. If you wanted to select the column ship_date in this case, the database wouldn't know which shipping date to choose for the whole group, so it would put just one, random value in the result.

To better understand the issue, take a look the following table:

order_id customer_id order_date ship_date total_sum (grouping) customer_id order_date sum(total_sum) ship_date
16 6 28-03-2015 29-03-2015 230.54 --> 6 28-03-2015 0 ?? 29-03-2015? 30-03-2015?
17 6 28-03-2015 30-03-2015 321.42 -->
18 6 28-03-2015 30-03-2015 2354.23 -->
19 6 29-03-2015 30-03-2015 4224.21 --> 6 29-03-2015 0 ??
20 6 29-03-2015 30-03-2015 2314.32 -->
21 6 29-03-2015 31-03-2015 124.21 -->
22 6 29-03-2015 31-02-2015 4125.32 -->
23 6 30-03-2015 03-04-2015 645.23 --> 6 30-03-2015 0 ??
24 6 30-03-2015 05-04-2015 7543.56 -->
25 6 30-03-2015 05-04-2015 2315.63 -->
26 7 02-04-2015 05-04-2015 523.98 --> 7 02-04-2015 0 ??
27 7 02-04-2015 06-04-2015 523.13 -->
28 7 02-04-2015 07-04-2015 8533.31 -->
29 7 03-04-2015 07-04-2015 4245.64 --> 7 03-04-2015 0 ??

## Exercise

Find the average salary for each employee. Show the last name, the first name and the average salary. Group the table by the last name and the first name.

### Stuck? Here's a hint!

Type:

SELECT
last_name,
first_name,
avg(salary)
FROM employees
GROUP BY last_name, first_name;