What is SQL? The lingua franca of data analysis

• Codecademy. Learn SQL. Free and interactive. Pro upgrade available for a fee.
• Khan Academy. Intro to SQL: Querying and Managing Data. Free video tutorial.
• SoloLearn. SQL Fundamentals. MySQL-oriented. Free.
• SQL Problems and Solutions and SQL Exercises. Interactive textbook and exercises. Free.
• SQLZoo. An interactive SQL tutorial developed and maintained by Edinburgh Napier University. Supports Microsoft SQL Server, Oracle Database, MySQL, IBM DB2, and PostgreSQL. Free. 
• Tutorialspoint. Learn SQL. Text only, not interactive. Free. 
• Udacity. Intro to Relational Databases. Uses Python and requires some Python knowledge. Free. 
• Udemy. Free courses include Introduction to Databases and SQL Querying, MySQL Database for Beginners, Microsoft SQL for Beginners, Hands-on SQL for Beginners (SELECT, FROM, and WHERE), and Sachin Quickly Learns (SQL).
• Vertabelo Academy. SQL Basics, Operating on Data in SQL, Creating Tables in SQL, and eight other interactive SQL courses. Some courses have free test drives, after which there may be a fee. There are seven additional courses for Microsoft SQL Server. The site also has a graphical database design tool for PostgreSQL, MySQL, Oracle Database, SQL Server, SQLite, and IBM DB2.
• W3Schools. SQL Tutorial. Free and available without registration.

Database documentation:

• MySQL documentation
• PostgreSQL documentation
• SQLite - SQL syntax
• IBM DB2 SQL reference
• Microsoft SQL Server technical documentation
• Oracle Database SQL reference

SQL commands quick reference:

• DDL: CREATE, ALTER, DROP (refer to tables)
• DMS: SELECT, INSERT, UPDATE, DELETE (refer to rows)
• DCL: GRANT, REVOKE (refer to permissions)