This is a nice book that helps anyone working with data the most important skills in SQL queries needed to pull information needed from database tables. Readers unfamiliar with this book can see what others have said here.
I implemented my solutions to these problems in SQLite rather than SQL Server. To get a copy of the Northwind database to work
the problems in I went here. Once the files were downloaded to create the
database I used the command:
sqlite3 Northwind.db < Northwind.Sqlite3.create.sql # creates the database
To learn this material as well as possible I worked through as many of the book's problems and exercises as I had time to.
To formalize my understanding I've put the SQL code in book form.
You can find the solutions I have done here.