SQL Child

About SQL Child

Learning to ask a database a question is easy enough. Learning to believe the answer, and to know when you should not, is the part nobody teaches you.

There is a moment that arrives for every beginner, usually late in the afternoon, when the answer finally appears. A number sits on the screen. It looks plausible. It has the right number of digits and it does not have an error attached to it. And in that moment you have to decide something that no manual will decide for you: do you believe it? Most people say yes, because the effort of getting there has already been considerable and because a number that appears looks a great deal like a number that is true. That decision, made quickly and quietly, is where nearly all real trouble begins.

This publication exists for that moment. Not for the mechanics of writing the request, which you can pick up from any tutorial in a weekend, but for the harder discipline of knowing what you actually asked and whether the shape of the answer matches the shape of the question. Those two things come apart more often than beginners expect. You can ask a perfectly well formed question and get a perfectly well formed answer to a slightly different question, and nothing anywhere will warn you. There is no red underline for a wrong idea.

The single hardest shift for a beginner is giving up the mental picture of moving through records one at a time.

Combining two collections of records sounds mechanical, but the mental picture you hold determines whether you get it right.

Narrowing your data at different moments gives different answers, and both of them will look entirely reasonable.

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