Section 30.3 Filtering with Aggregated Results
If we want to answer the question “Which stations were the starting point for 10,000 or more rides?”, it would seem like we want to write something like this:
SELECT
start_station,
COUNT(*) AS n_trips
FROM
trip_data
WHERE
n_trips > 10000
GROUP BY
start_station
Unfortunately, you can never include an aggregating function or result within the WHERE clause. Trying to do so will give an error. However, there is a HAVING clause that can be used just like a WHERE to filter data, only it works with aggregations. To ask about the stations with over 10000 trips we would write:
As with the ORDER BY clause, HAVING is an operation on the results. While WHERE filters the data being queried, HAVING filters the results based on the value of an aggregating function. The HAVING clause can only be used immediately following the GROUP BY clause.
Checkpoint 30.3.1.
Below is a query to find the total time each bike was ridden. Add a HAVING clause to select only the bikes that were used for less than 30000 minutes.
Hint.
The HAVING must come before the ORDER BY
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