SQL

The intention here is to describe ANSI SQL, rather than vendor specific SQL.

You can use SQL Fiddle to try out SQL syntax, the site supports MySQL, Oracle, PostgreSQL, SQLite and MS SQL Server and hence serves as a useful test but there is nothing like a proper database.

Joins

A good visual starting point is Visual Representation of SQL Joins - CodeProject

Where Clause

It is easy to forget how "BETWEEN" works, so, for clarity it is this:

WHERE Length BETWEEN 5 AND 10
WHERE Length >= 5 AND Length <= 10
It is also worth noting that BETWEEN works with "WHERE" and "HAVING".