MySQL - How Do I Count Nulls and Not Nulls?

Source: https://stackoverflow.com/questions/9245038/mysql-how-do-i-count-nulls-and-not-nulls

SELECT prod_code,
       COUNT(email) AS total_installs,
       COUNT(install_slot) AS used_installs
FROM installs
WHERE email='example@example.com'
GROUP BY prod_code

COUNT counts NOT NULL values only.