I found these are useful:

• SQLBolt and SQLZoo

  Interactive exercises to learn SQL.

• The Database Programmer

  Blog with all things related to database applications.

• modernSQL

  SQL explained in simpler terms.

• SQL for smarties

  Advanced SQL, bible.

CMU Database Groupo has a YouTube channel.

Performance

• SQL Performance Explained

• SQL Indexing and Tuning

  When to apply index.

• High Performance MySQL

MySQL

• MySQL official documentations

PostgreSQL

• PostgreSQL official documentations
• Mastering PostgreSQL
• PostgreSQL Exercises

I have some fragments on PostgreSQL, check it out!