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Last updated 2017-08-07

Databases

What is a Database?

General

A database is any collection of related data.

Restrictive

A database is a persistent, logically coherent collection of inherently meaningful data, relevant to some aspects of the real world.

What is a Database Management System?

A database management system (DBMS) is a collection of programs that enables users to create and maintain a database. According to the ANSI/SPARC DBMS Report (1977), a DBMS should be envisioned as a multi-layered system.

What Does a DBMS Do?

Database management systems provide several functions in addition to simple file management:

  • allow concurrency
  • control security
  • maintain data integrity
  • provide for backup and recovery
  • control redundancy
  • allow data independence
  • provide non-procedural query language
  • perform automatic query optimization

What is a relational database?

• a database that treats all of its data as a collection of relations

What is a relation?

  • a kind of set
  • a subset of a Cartesian product
  • an unordered set of ordered tuples

Relational Database Operators

Data models consist of data structures and permitted operations on those data structures. Part of Codd’s genius was to recognize that many of the standard set operators that can take relations as operands map nicely to real data manipulation problems:

• Cartesian product
• union
• intersection
• difference

Codd devised some additional operators to provide extra manipulatory power:

• select
• project
• join
• divide

The operators have now been extended to include more useful manipulations:

• outer join
• outer union

Relational Database Normal Forms

First Normal Form:

A relation is in first normal form (1NF) if and only if all underlying domains contain atomic values only.

Second Normal Form:

A relation is in second normal form (2NF) if and only if it is in 1NF and every non-key attribute is fully dependent on the primary key.

Third Normal Form

A relation is in third normal form (3NF) if and only if it is in 2 NF and the nonkey attributes are mutually independent.

Read more on http://www.esp.org/db-fund.pdf