Материалы:

SQL:
1) https://sqlbolt.com/
2) https://sql-academy.org/ru
3) https://stepik.org/course/63054/promo
4) https://www.sql-ex.ru/

Python:
1) https://www.youtube.com/watch?v=3Xc3CA655Y4
2) https://www.youtube.com/watch?v=vmEHCJofslg&t=1s
3) https://www.youtube.com/watch?v=3Xc3CA655Y4

A/B test:
1) https://habr.com/en/post/242701/