Understanding SQL Joins
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Left join. Right join. Inner join. If you've ever wondered what all this jargon means, it's time to find out. Welcome to the wild, the wacky, the insanely cool world of SQL joins.A few years ago, I decided to visit Paris for a short two-week holiday. I didn't know French, had never visited the country before, and only knew that it was famous for its wines and cheeses. However, I naively assumed that I'd have no trouble getting around - after all, I reasoned, Paris was a cosmopolitan city, and English was bound to be one of the more common languages there.
As you might imagine, my naïve optimism came crashing down to earth the moment I got off the plane. All the signs were in French, everyone seemed to be gesticulating frantically, and I couldn't understand a single word of what the people around me were saying. Sign language got me a cab, and a hotel booking slip got me to a bed - but I quickly realized that if I was going to survive in the city, I'd need to learn a little French...and fast!
With a little help from some friendly students, I quickly learnt the basics - "bon jour" when you meet someone, "au revoir" when you part, "merci" when you want to thank someone and - if all else fails - "parlez-vouz anglais?". By the end of my holiday, I understood a great deal more of what was happening around me, and was even able to conduct rudimentary conversations with the locals.
What does this have to do with anything? Quite a lot.
You see, learning a programming language is a lot like learning a spoken language. You start off knowing almost nothing and slowly work your way into a situation where you're comfortable with using it to accomplish basic tasks. You then build on this basic knowledge in order to acquire greater proficiency with the language, until your versatility and complete comfort with the language's most arcane nuances impress even your closest friends.
If you've been following this column over the past few months, you've probably picked up the basics of SQL, the standard language used to communicate with databases; I've used it frequently when discussing database-driven, dynamic Web applications. Even if you started out as a novice, you're probably now fairly comfortable using SQL to retrieve data from a database or insert new records into it. And so, it's time to begin the second phase of your education - learning some of the more advanced things you can do with SQL.
That's where this article comes in. Over the next few pages, I will be introducing you to joins, which have a reputation for being complex, difficult to understand and frightening. Most of this is just nasty rumour - they're actually simple, extremely friendly and nowhere near as frightening as some of the bills I got in Paris. Let me show you.
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