Normalisation Notes

SQL Interview: Normalisation, NULL and Query Optimisation

Design and correctness questions: the normal forms, why NULL breaks intuition, and how to answer "this query is slow, what do you do?"

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Anomalies and Denormalisation

The three anomalies normalisation prevents, and the disciplined way to reintroduce redundancy when reads genuinely demand it.

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BCNF and Functional Dependencies

The formal notation behind normalisation, and the stricter normal form that catches what 3NF allows through when candidate keys overlap.

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First, Second and Third Normal Form

Normalisation worked through on one table: remove repeating groups, then partial dependencies, then transitive ones - with the SQL at each step.

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Data Integrity, Redundancy and Consistency

The three kinds of integrity a relational database enforces, why redundancy causes anomalies, and where a derived value is worth its maintenance cost.