"Introduction to Database System"----relational database (i)

2.1 Relational data structure and formal definition

Relational database system is a database system that supports relational model.

The relational model consists of three parts: relational data structure, relational operation set and relationship integrity constraint.

2.1.1 Relationship

The data structure of the relational model is very simple and contains only a single data structure-----relationship.

1. Domain: A collection of values that have a system data type

2. Cartesian product:

For example, a={a,b},b={0,1,2}, the axb={<a,o>,<a,1>,<a,2>,<b,0>,<b,1>,<b,2>,}<a,0 > is a tuple

A is a component

3. The relationship:<a,0> is called the relationship on the domain a*b, the number of a*b subsets is called the goal or the degree.

n = 1 The relationship is called a cell relationship

n = 2 The relationship is called a two-tuple relationship

A relationship is a two-dimensional table, with each column corresponding to a name, called a property.

An attribute group that uniquely identifies a tuple is called a candidate code.

If there are multiple candidates in the relationship, select one of the main codes.

The properties of the candidate code are called main attributes, others are called non-primary or non-code attributes.

In the simplest case, the candidate code contains only one attribute, in extreme cases all attributes are candidate codes, called full yards.

There are three types of relationships:

Basic relationship (base table or base table): Is the actual table, and he is the logical representation of the actual stored data.

Query table: The table that corresponds to the query result

View chart: A table that is exported by a base table or other icon, which is a virtual table

2.1.2 Relationship Mode

Relationship is actually a two-dimensional table, the description of the relationship is called the relationship pattern.

A relationship is the state or content of a relationship pattern at a certain point in time. Relationship patterns are static, stable, and relationships are dynamic, changing over time

2.1.3 Relational database

Relational databases include:

Definition of several domains

Several relationship patterns defined on these domains

The value of a relational database is the set of relationships that these relational patterns correspond to at some point, often referred to as relational databases.

"Introduction to Database System"----relational database (i)