FAQ: Conditional Aggregates - COUNT(CASE WHEN )

This community-built FAQ covers the “COUNT(CASE WHEN )” exercise from the lesson “Conditional Aggregates”.

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This exercise can be found in the following Codecademy content:

SQL: Table Transformation

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I think that this exercise can also be done using subqueries as in the first lesson:

SELECT state,
   (SELECT COUNT(*)
   FROM airports AS a
   WHERE a.elevation < 1000
   AND a.state = airports.state) AS count_low_elevation_airports
FROM airports
GROUP BY state;

Is there any reason to prefer using COUNT(CASE WHEN) over this?

Your code doesn’t count blank states

Could anyone explain me the logic of the query ? how does “state” and counted elevations connect if we don’t use ‘where’ statement ?

The GROUP BY relates the COUNT with the ‘state’ column. The table has a ‘state’ value and ‘elevation’ value for each row. So they are already “connected”. The query tells you how many airports in each state are considered “low elevation”. without the GROUP BY you just get a total of all low elevation airports.

Does anybody else feel that this particular course hasn’t really helped them learn SQL, its just copying code and changing it slightly. Highly doubt this is how its done in the real world lol. Its been good up to now