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...
#Relational Model
Relations, tuples, domains and relational integrity.
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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
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
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...
DBMS
Relation Schema and Relation Instance
A relation schema is the fixed definition of a relation; a relation instance is the set of tuples in it right now. A relational database schema is the...
DBMS
Relations, Tuples, Attributes and Domains
The relational model has exactly four structural terms. A relation is a set of tuples, a tuple is one fact, an attribute is a named column and a domai...
DBMS
Converting an ER Diagram into Relational Tables
Seven rules turn any ER diagram into a relational schema: entities become tables, one to many puts a foreign key on the many side, many to many become...
DBMS
The Relational Model as a Data Model
The relational model stores everything in tables and connects them by matching values rather than pointers. That one decision delivers declarative que...