18.3. 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:

SELECT start_station, COUNT(*) AS n_trips -- call results of aggregation n_trips FROM trip_data GROUP BY start_station HAVING n_trips > 10000 -- filter the results using the COUNTs calculated ORDER BY n_trips DESC

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.

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 1000 minutes.

SELECT bike_number, SUM(duration) AS total_time FROM trip_data GROUP BY bike_number ORDER BY total_time DESC ==== assert 0,0 == W01236 assert 0,1 == 925 assert 1,0 == W00053 assert 1,1 == 537

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