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...
#Intermediate
Material for readers past the basics.
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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
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
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
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
Attribute Closure, Step by Step
The closure of an attribute set is everything it determines. Computing it is a short mechanical loop, and it is the single tool used to find keys, tes...
DBMS
Functional Dependencies Explained
A functional dependency says that whenever two tuples agree on X they must also agree on Y. It is a rule about every possible instance, not an observa...
DBMS
Join Operations in Relational Algebra
Theta join, equi join and natural join, all defined as a selection over a Cartesian product. The natural join joins on every common attribute and remo...
DBMS
Set Operations in Relational Algebra
Union, intersection and difference come straight from set theory, but they require union compatibility. Cartesian product combines every pair and is t...
DBMS
Relational Algebra: Selection, Projection and Rename
The three unary operators. Selection picks rows, projection picks columns and removes duplicates, and rename makes an expression reusable. Together th...
DBMS
Null Values and Three Valued Logic
Null means the absence of a value, not zero and not empty. Comparisons with null produce unknown, which turns two valued logic into three and changes...
DBMS
Foreign Keys and Referential Actions
What happens to child rows when a referenced parent row is deleted or updated. Cascade, restrict, set null, set default and no action, with the rule f...