# Group-Based Aggregation

Learn to group data.

## We'll cover the following

## Why do we need to group?

This is going to be a little bit more difficult, but we’ll answer this question in small steps.

If we list the `orders_items`

, we see that we have the `price`

and the `quantity`

columns.

```
mysql> select * from order_items;
+----+----------+------------+-------+----------+
| id | order_id | product_id | price | quantity |
+----+----------+------------+-------+----------+
| 4 | 2 | 1 | 50.00 | 2 |
| 5 | 3 | 1 | 50.00 | 1 |
| 6 | 3 | 2 | 19.80 | 1 |
| 7 | 4 | 2 | 19.80 | 5 |
| 8 | 5 | 3 | 5.00 | 1 |
| 9 | 6 | 3 | 5.00 | 2 |
| 10 | 7 | 2 | 19.80 | 1 |
+----+----------+------------+-------+----------+
7 rows in set (0.00 sec)
```

The total amount of the first order with an `id`

value of `2`

is 2 x 50.00 = 100.00. The total amount of the order with an `id`

value of `3`

is 1 x 50.00 + 1 x 19.80 = 69.80, and the total amount of the order with an `id`

value of `4`

is 5 x 19.80 = 99.00.

This means that the `order_items`

table has all the necessary information to build a result set like the following.

```
mysql>
+----------+------------------+
| order_id | amount_per_order |
+----------+------------------+
| 2 | 100.00 |
| 3 | 69.80 |
| 4 | 99.00 |
| 5 | 5.00 |
| 6 | 10.00 |
| 7 | 19.80 |
+----------+------------------+
6 rows in set (0.01 sec)
```

How can we get there? Let’s try the following first:

```
mysql> select order_items.order_id, order_items.price * order_items.quantity as amount_per_order_item from order_items;
+----------+-----------------------+
| order_id | amount_per_order_item |
+----------+-----------------------+
| 2 | 100.00 |
| 3 | 50.00 |
| 3 | 19.80 |
| 4 | 99.00 |
| 5 | 5.00 |
| 6 | 10.00 |
| 7 | 19.80 |
+----------+-----------------------+
7 rows in set (0.00 sec)
```

The result set above doesn’t differ much from the following:

```
mysql> select * from order_items;
+----+----------+------------+-------+----------+
| id | order_id | product_id | price | quantity |
+----+----------+------------+-------+----------+
| 4 | 2 | 1 | 50.00 | 2 |
| 5 | 3 | 1 | 50.00 | 1 |
| 6 | 3 | 2 | 19.80 | 1 |
| 7 | 4 | 2 | 19.80 | 5 |
| 8 | 5 | 3 | 5.00 | 1 |
| 9 | 6 | 3 | 5.00 | 2 |
| 10 | 7 | 2 | 19.80 | 1 |
+----+----------+------------+-------+----------+
7 rows in set (0.00 sec)
```

However, there are a few differences, as listed below:

- It doesn’t display the
`id`

column. - It doesn’t display the
`product_id`

column. - The two
`price`

and`quantity`

columns have been multiplied and now give the`amount_per_order_item`

.

As we can see, the snippet `order_items.price * order_items.quantity as amount_per_order_item`

multiplies the values of the two columns and sets a label to the result. This is why the new derived-value column is displayed with the label `amount_per_order_item`

.

Cool! Now, let’s try another concept:

```
mysql> select sum(price), sum(quantity) from order_items;
+------------+---------------+
| sum(price) | sum(quantity) |
+------------+---------------+
| 169.40 | 13 |
+------------+---------------+
1 row in set (0.01 sec)
```

The `sum()`

function can be used on a column. Then, SQL will sum the values of that column for all the rows that match the result set criteria. The `sum()`

is an aggregate SQL function. We’ll learn more about that later on.

## Show the total quantity of each order

```
mysql> select order_items.order_id, sum(price), sum(quantity) from order_items group by order_id;
+----------+------------+---------------+
| order_id | sum(price) | sum(quantity) |
+----------+------------+---------------+
| 2 | 50.00 | 2 |
| 3 | 69.80 | 2 |
| 4 | 19.80 | 5 |
| 5 | 5.00 | 1 |
| 6 | 5.00 | 2 |
| 7 | 19.80 | 1 |
+----------+------------+---------------+
6 rows in set (0.00 sec)
```

This is even better because it applies the aggregate functions (the two `sum()`

function calls) to all rows that match the criteria, but separately for each `order_id`

. Double-check the amounts. Do the `prices`

of `order_id 2`

sum up to `50.00`

? Do the `prices`

of `order_id 3`

sum up to `69.80`

? What about the sums on the `quantity`

columns? Do they match?

It seems that we’re getting closer and closer to our target. We’re only left to combine the information from `price`

and `quantity`

before summing up. The combination needs to be a multiplication.

## Show the total amount of each order

```
mysql> select order_items.order_id, sum(price * quantity) as amount_per_order from order_items group by order_id;
+----------+------------------+
| order_id | amount_per_order |
+----------+------------------+
| 2 | 100.00 |
| 3 | 69.80 |
| 4 | 99.00 |
| 5 | 5.00 |
| 6 | 10.00 |
| 7 | 19.80 |
+----------+------------------+
6 rows in set (0.00 sec)
```

Perfect! We now display the sums of prices multiplied by quantities, but we limit the summation/aggregation to the matching `order_ids`

so that there’s one summation/aggregation for each `order_id`

.

Practice in the given widget below for better understanding:

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