Some correspondences between Microsoft operating systems (wiki DOS, Win) and Star Wars movies (wiki).
As you can see, the glorious and victorious episodes 6 and 3 are the most stimulating for Windows production but when old versions are Jedi-inspired, the recent versions are on the dark side of the Force.
Better to see once than to hear a hundred times. This small article explains what are the different kinds of joins in SQL.
At the first time you should take into account that SQL is a declarative 4th generation language. So the best way to use it (especially for beginners) is to write the query and let SQL optimizer do its job. That means thinking in set-oriented terms and avoiding sub-queries.
Let's take two example tables: Companies and Contacts.
Some countries have integer numbers printed on the transport tickets, for example, in ex-USSR. The ticket is "happy" when the sum of the first half of digits equals to the last half one. Then you should eat it and make a wish.