# 9.7. Aggregation or Group By¶

One very powerful feature of SQL is that it allows us to create summary information by grouping rows together. For example, we could ask ourselves how many bike trips were taken for each subscriber type, and which subscriber type has the most bike trips.

GROUP BY member_type takes all the rows with a given member_type and produces a single row in the result. This means that we need to tell SQL how we want to combine the other columns’ values into a single row. The above example uses COUNT(*) which reports the number of rows that were combined.

Aggregating the values for member_type is not hard, since they are all the same, SQL just gives us a single copy of the publisher name. Other columns, we need to either ignore (causing them to be omitted from the output) or specify a way to aggregate them.

We must specify an aggregate function for any column that we SELECT in our query (except the column that we’re grouping by) in order for the command to succeed. If we don’t specify a way to aggregate the value most database servers will complain. However, SQLITE does not. SQLite lets you do silly things without giving you an error. For example, the following query will work, but you have no idea what the results actually mean.

Here you have grouped by member_type, but without member_type in the select clause you have no idea which rows correspond to which member type. That is why most databases will flag this as an error. Furthermore, the duration field may be the first duration in the group or maybe the last duration in the group or possibly in between, But it’s not defined. The best practices for writing group by queries that work well across database systems are as follows:

• Always include the GROUP BY column(s) in your SELECT clause.

• If you include a column that is not in the GROUP BY clause in your SELECT clause you must do some form of aggregation on the values in that column. For example, min, max, mean, count, etc.

Let’s go back briefly to the first query in the Aggregation section. The top result was the count of bike trips for member_type Member:

member_type

COUNT(*)

Member

979814

If you’d like to get a more granular break down of the count, you may specify multiple columns to aggregate within the GROUP BY clause, for example: further breakdown the aggregate count by the start station IDs:

Great! Now that you’re familiar with how to aggregate data using SQL query by using COUNT() as your aggregation function, let’ take a look at other aggregation functions.

There are many such functions. Some common ones include:

• SUM: To add the values together

• AVG: To compute the mean of the values

• MIN or MAX: To compute the minimum and maximum respectively

So we could for example compute the total number of minutes of all bike trips for all subscriber types

## 9.7.1. Practice Exercises¶

Compute the average duration of bike trips for each starting station id and list the results in order of highest average to lowest average for the 10 stations with the highest average. What is the highest average duration?

What is the bike_number and count of the bike with the most rides?

How many total rides by Members and Casual users?

What is the station that has the most rides that start and end at the same station? How many rides started there?

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