DBMS
Keys in the Relational Model
Super key, candidate key, primary key, alternate key, composite key, foreign key and surrogate key, defined formally with a worked method for finding...
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DBMS
Entity, Referential and Domain Integrity
The three integrity rules of the relational model: a primary key may not be null, a foreign key must match an existing primary key or be null, and eve...
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...
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
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
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
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
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
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
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
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
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...