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 rowsSUM
adds the values of a numeric columnMIN
calculates the minimum of a numeric columnMAX
calculates the maximum of a numeric columnAVG
calculates the mean of a numeric columnMEDIAN
calculates the median (middle value) of a numeric columnMODE
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 calculates 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 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.