Answer 1

Вот несколько отличных ресурсов для изучения SQL онлайн, надеюсь, вы тоже можете помочь:
Stanford - Introduction to Databases (Coursera)
SQL exercises
SQLZOO
Database eLearning
PostgreSQL tutorial
PostgreSQL exercises
The Schemaverse is a game, I never played it so I can't tell much
SQLite
Sams Teach Yourself SQL in 10 Minutes
Learn SQL The hardway - I like this text book a lot.
SQLRU
SQLCourse
( also w3schools has a good sql playground )
Found this on Quora: looked good

Answer 2

добро пожаловать

Answer 3

спасибо!!!