DBMS
Armstrong Axioms and Inference Rules
Three primary axioms - reflexivity, augmentation and transitivity - are sound and complete, meaning they derive every dependency that follows and none...
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DBMS
Finding All Candidate Keys of a Relation
A reliable method for finding every candidate key: classify attributes by where they appear in the dependencies, build a compulsory core, then extend...
DBMS
Equivalence of FD Sets and Minimal Cover
Two dependency sets are equivalent when each implies the other. A minimal cover is the smallest equivalent set, computed in three steps, and it is wha...
DBMS
Why Normalisation Exists: Anomalies and Redundancy
Redundancy causes three anomalies: you cannot insert a fact without an unrelated one, an update must change many rows, and a delete destroys informati...
DBMS
First Normal Form
A relation is in 1NF when every value is atomic and there are no repeating groups. It is the entry requirement for the relational model itself, not me...
DBMS
Second Normal Form and Partial Dependency
A relation is in 2NF when it is in 1NF and no non prime attribute depends on only part of a candidate key. It only ever matters when the key is compos...
DBMS
Third Normal Form and Transitive Dependency
A relation is in 3NF when it is in 2NF and no non prime attribute depends on another non prime attribute. It is the practical target for most producti...
DBMS
BCNF and BCNF Decomposition
BCNF requires every determinant to be a super key. It is stricter than 3NF, removes the last key based redundancy, and sometimes cannot be reached wit...
DBMS
4NF, 5NF, Multivalued and Join Dependencies
Fourth normal form removes independent multivalued facts stored in one relation. Fifth normal form removes join dependencies, where a relation can be...
DBMS
Lossless Decomposition and Dependency Preservation
A decomposition is lossless when joining the pieces reproduces the original exactly, and dependency preserving when every dependency can still be chec...
DBMS
Complete Worked Normalisation: Unnormalised to BCNF
One realistic table taken all the way from unnormalised form through 1NF, 2NF, 3NF and BCNF, with every dependency, key and decomposition shown at eac...
DBMS
Denormalisation and When to Use It
Denormalisation deliberately reintroduces redundancy to make reads faster. It is a considered trade, valid only with evidence, and it always transfers...