How to create fast database queries

Archive for July 12th, 2009

Double-thinking in SQL

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One of the first things a novice SQL developer learns about is called thinking in SQL, which is usually being opposed to procedural thinking

Let's see what part of brain does this intellectual activity take and how to use it.

Two features distinguish SQL from other languages you learned as a 11-year old kid on your first PC, like BASIC or perl or maybe even C++ if you're such a Wunderkind.

First, SQL is set-based. It does things with sets.

Every tool is designed to do things with something else. Like, you use a hammer to do things with nails, or use a screwdriver to do things with screws, or use an oven to do things with food.

Same with computer languages.

BASIC does things with variables. perl does things with scalars, arrays, hashes and file streams. Assembly does things with registers and memory.

You should not be confused by something like registers are just a special case of variables, or a hash is just a generalized container which exposes this and this method or something like that. No.

A hash is a hash, a variable is a variable and a register is a register.

Like, an egg is a food and rice is a food and it's possible to cook some eggs in a rice cooker and vice versa, but they are just wrong tools to do that.

Akihabara district in Tokyo City. You can buy any type of gadget you can imagine, which is about 2% of what you will find there.

Akihabara district in Tokyo City. You can buy any type of gadget you can imagine, which is about 2% of what you will find there.

Prehistoric men had to make do with hammerstones and lithic blades (even to court their women), but now we have a whole district in Tokyo City for gadgets with USB type A, and another district for gadgets with USB type B.

So if you feel the urge to hash something and then make a good old array out of this, you don't use assembly, but perl or PHP instead.

Same with SQL. SQL does things with sets.

It's a tool that allows you to take a dozen or two of sets, mix them together, knead and wedge them then chop them apart and mix again, but the output you get is still a set and all inputs are sets.

Eeverything you do in SQL, you do it on sets. That's why SQL is called a set-oriented language.

Ok, that was the first feature that distinguishes SQL from other languages. What's the second one?

SQL is a declarative language. This means that you express what you want to do with sets, not how you want to do it.

This requires a little explanation.
Read the rest of this entry »

Written by Quassnoi

July 12th, 2009 at 11:00 pm

Posted in Miscellaneous