SQL is not hard at all. It’s just all about syntax. You need to learn two parts: SQL basic commands (such as join, group by, order by) and SQL window function (such as row_number(), partition by).

Tutorials:

• Udemy SQL intro course. It covers all basic SQL commands. I knew nothing about SQL when I took the course, so I learned a lot as a beginner. If you’re already familiar with SQL. You can go ahead and use the resources below to practice SQL product questions.
• Mode Analytics. I have not checked this one but lots of people recommended to read the tutorial. It also covers window functions.
• W3School. A good place to check for syntax and example of SQL command.

Practice SQL questions:

• Leetcode. ~ 120 questions
• Hackerrank

If you don’t feel like installing SQL on your local machine. SQLfiddle is a good place to experiment and build your schema.