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)