SQL

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http://sqlfiddle.com/about.html

http://sol.gfxile.net/g3/

https://schemaverse.com

https://lagunita.stanford.edu/courses/Home/Databases/Engineering/about

http://www.mysqltutorial.org

https://www.sololearn.com/Course/SQL/

https://learncodethehardway.org/sql/

https://code.tutsplus.com/articles/sql-for-beginners--net-8200

http://sqlzoo.net https://www.khanacademy.org/computing/computer-programming/sql

https://www.hackerrank.com/domains/sql/select

http://www.sqlcourse.com

https://www.topzenith.com/2017/07/top-10-best-sql-video-tutorials.html

https://academy.vertabelo.com/blog/18-best-online-resources-for-learning-sql-and-database-concepts/

http://radacad.com/basics-of-m-power-query-formula-language

https://community.modeanalytics.com/sql/tutorial/introduction-to-sql/