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...
#Interview
Interview preparation material.
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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
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...
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
Building Relational Algebra Expressions
How to turn an English question into an algebra expression, read an expression tree, and apply the equivalence rules an optimiser uses to make the sam...
DBMS
The Division Operator
Division answers questions containing the word every. It is the hardest operator to recognise and the easiest to verify, and it can always be rewritte...
DBMS
Outer Joins, Semi Join and Anti Join
Inner joins discard unmatched tuples. Outer joins keep them and pad with nulls. Semi join keeps only the left tuples that matched, and anti join keeps...
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...