# 18.1. Aggregating Data¶

Say we want data about how many rides lasted longer than an hour. At this point we
don’t care about where they started, or which bike they were on, we just want to
know how many such rides there were.
To ask for the total number of records a query finds, we can select `COUNT(*)`

.

The `AS`

keyword is used after `COUNT(*)`

to give the data that is returned a name. If we
leave it off, the “column” of data will just be labeled `COUNT(*)`

. Calling it `number_rides`

makes it much clearer what we are looking at.

How many trips started at station 31111?

Hint: Build your way to the final query. Start by selecting all the data (`*`

) for all the
trips. Then write a `WHERE`

to only select ones that start from 31111. Finally, instead of selecting
all of the columns, select just the count of the number of records.

`COUNT`

is one of the aggregation functions provided by SQL. **Aggregation** is the process
of combining data and `COUNT`

combines all the records and tells us how many there are.
But there are other ways we can aggregate data with SQL:

`COUNT`

counts the rows`SUM`

adds the values of a numeric column`MIN`

calculates the minimum of a numeric column`MAX`

calculates the maximum of a numeric column`AVG`

calculates the mean of a numeric column`MEDIAN`

calculates the median (middle value) of a numeric column`MODE`

calculates the mode (most common value) of a numeric column

For example, the query below calculates the minimum and maximum trip duration:

Write a query to calculate the mean duration (average) of all trips.

You use `WHERE`

filtering with aggregating functions. The
query below calculates the longest trip duration just for trips made by Casual
member type users.

Write a query to get the total duration (sum) of all trips taken on bike W01274.

## 18.1.1. Counting Unique Values¶

It is often helpful to not only count the number of rows but also to count the number
of unique values of a column. You can do this using the `DISTINCT`

keyword.

To count the distinct values of a column, you can simply use `COUNT`

along
with `DISTINCT`

. For example, the query below counts the number of bike
numbers used.

Write a query to get the number of unique start locations there are in the table.