To maintain my SQL skills I regularly practice them. I use the following sites:
https://sqlbolt.com/
https://sqlzoo.net/
http://www.sql-ex.ru/
http://www.sql-tutorial.ru/ru/book_sql_tips_and_solutions_10.html
http://sql-tutorial.ru/ru/book_computers_database.html
https://www.pgexercises.com/
http://www.sqlcourse.com/
online books with exercises
http://rsdn.org/res/book/db/sqltraining.xml
not much explanation here, only forums:
https://www.hackerrank.com/domains/sql/select
Example of technical interview:
https://habr.com/ru/post/181033/
https://www.youth4work.com/onlinetalenttest/Test-SQL-Server
