Calculating percentages or rates is fundamental to data analysis of any kind. We should feel it by heart, use it as easy as addition or subtraction.
I find the last exercise a perfect material to repeat all the ways we’ve learned so far to calculate percentages.
Percentage with CASE
We know that for every user record there’s at least one record in devices
table. It allows us to calculate percentage of users with a certain device_type
only based on devices
table:
SELECT 100.0 * COUNT(DISTINCT(CASE WHEN device_type = 'iphone' THEN user_id END)) /...
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About SQL Habit
Hi, it’s Anatoli, the author of SQL Habit. 
SQL Habit is a course (or, as some of the students say, “business simulator”). It’s based on a story of a fictional startup called Bindle. You’ll play a role of their Data Analyst
and solve real-life challenges from Business, Marketing, and Product Management.
SQL Habit course is made of bite-sized lessons (you’re looking at one atm) and exercises. They always have a real-life setting and detailed explanations. You can immediately apply everything you’ve learned at work. 