count-the-number-of-trues-in-an-aggregate-query.md

Count The Number Of Trues In An Aggregate Query

The sum function is an aggregate function that allows you to sum up a bunch of integers. What if you want to sum up a boolean column? You may want to know how many times true appears in a collection of grouped records.

This can be done by mixing in a case statement.

select
  author_id,
  sum(case when available then 1 else 0 end)
from books
group by author_id;

Here, we are able to find out for each author how many books they have available.

If we want to count false values, we can just invert the sum statement:

sum(case when available then 0 else 1 end)

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