17.6. Sorting¶
We can use SQL to specify how to sort the records that are retrieved by a query using
an ORDER BY clause. For example, the following query returns bike trips ordered by
duration:
The column name after the ORDER BY clause specifies which column to use as
the sorting key, in this case duration. The shortest trips are listed first and the longest
trips are listed last. This increasing, or ascending order is the default.
To make the order go from largest to smallest (descending order), we need to
specify DESC after the column name we are ordering by. (If we want to be explicit
about ascending order, we can say ASC, but do not need to since that is the default.)
17.6.1. LIMIT¶
That query returns over 400,000 records (the environment we use will only show us 100
of those to prevent the web page from being overwhelmed with data). If we only want to
see the 10 longest trips, we can use LIMIT to specify the maximum number of rows to
return:
but to LIMIT the
output to the first 10 rows.
We can combine ORDER BY and LIMIT with the other clauses we have learned about
to do something like selecting just the start date, station, and the duration for only
trips starting from station 31111, sorting them from longest to shortest, and only
keeping the first (longest) 10:
The ordering of the clauses is important. The WHERE must come before the ORDER BY
which should come before the LIMIT.
17.6.2. Multiple Sorts¶
In SQL, you can also order by multiple columns. In this case, the resulting rows
will be ordered by the values in the specified columns, ordered by the first
specified column, then the second specified column, and so forth. For example,
if you ORDER BY start_station, end_station, duration, the rows will first
be sorted by start_station, then within each value of start_station the
rows will be ordered by end_station, then within each pair of
start_station and end_station values, the rows will be ordered by
duration.
Now try to write SQL queries that will answer the following questions.
On what bike number was the most recent (by start date) trip?
Select just the column that has the bike number, and sort the results so that
the most recently started trip is first. It is easiest to begin by selecting all
data (*) while you get the query logic correct so you can see if the data is
sorted correctly. Once you get it working, select just the column with the bike number.
and limit the results to one record.
Write a query using ORDER BY to find the starting station and duration of
the longest ride that started and ended at the same station.
Again, start simple and build your way to the correct query. Start by selecting
all columns so you can see what data you are getting as you work on the WHERE and
ORDER BY.
Hint: both sides of the WHERE’s condition can be column names. If I wanted to find
a record where the start and end date were the same I could do:
WHERE start_date == end_date