Introduction to database system - Chapter 2 - relational database (2.1~2.3) (important knowledge points)

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Preface

take 【 Introduction to Database System ---- Chapter two – relational database (2.1~2.3)】 The important knowledge points in

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1. Relationship

The relation is a finite subset of Cartesian product .
The relation is a two-dimensional table .
Cartesian product itself is meaningless in physical space , The relation is to extract a meaningful subset from the Cartesian product .

• Related terms ：
  • Objective ( degree )： There are several columns in the table for several degrees or items .
  • There are several behaviors and several relationships .
  • Each row of the table corresponds to a tuple .
  • Each column of the table corresponds to a field , Each column of the table corresponds to an attribute ,n Objective relationship n Attributes .
  • Candidate code ： If an attribute in the relationship can uniquely identify a tuple , Call this attribute a candidate code .
  • Main code ： If a relation has more than one candidate code , Select one of the main codes .
  • Main attribute ： The attribute of the candidate code is the primary attribute .
  • Non primary property ( Or non code properties )： Attributes that do not contain any candidate codes are non primary attributes or non code attributes .
  • Full size ： If all the attributes of a relational schema are candidates for this relational schema , Full code .
• There are three types of relationships , Basic relationship ( It is also called basic table or base table )、 Query table 、 View table .
  • The base table is the actual table , Is the logical representation of the actual stored data .
  • The query table is the table corresponding to the query result .
  • A view table is a table derived from a base table or other view table , It's a virtual watch , Not the data that should actually be stored .

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2. The nature of the basic relationship

1. Columns are homogeneous , That is, the components in each column are the same type of data , From the same domain .
2. Different columns can come from the same domain , Call each of these columns an attribute , Different attributes should be given different attribute names .( Property name uniqueness ).
3. The order of the columns doesn't matter , That is, the order of columns can be exchanged arbitrarily .
4. Two candidate codes of... Cannot take the same value .
5. The order of the lines doesn't matter , That is, the order of lines can be exchanged arbitrarily .
6. The component must be atomic , That is, each component must take an inseparable data item ( Cannot have table in table ).

The relationship model requires that the relationship must be normalized , That is, the relationship must meet certain normative conditions . The most basic of these normative conditions is , Each component of the relationship must be an indivisible data item . Normalized relationships are referred to as paradigms .

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3. Relationship model

The description of a relationship is called a relational schema .

• Formal representation of relational schema ：R(U, D, DOM, F)
  • R Name the relationship
  • U A collection of attribute names that make up the relationship
  • D by U The domain from which the attribute in the
  • DOM A collection of images to the domain for the attribute ( The values in each row correspond to the values in the field )
  • F Is the dependency set of data between attributes
• Relational patterns are usually abbreviated to R(U) or R(A1,A2,...) among A_i For the property name .
• The relationship pattern is static 、 The stability of the , And relationships are dynamic 、 Changing over time .

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4. Relationship operation

• Characteristics of relationship operation ： Set operation mode , That is, the objects and results of the operation are collections .
• Common relational operations ：
  • Inquire about ( The most important part of relationship operation )：
    Query operations can be divided into selection 、 Projection 、 Connect 、 except 、 and 、 Bad 、 hand over 、 Cartesian product , Selection 、 Projection 、 and 、 Bad 、 Cartesian product is 5 Basic operations , Other operations can be defined and exported with basic operations .
  • Data update operation ：
    Insert 、 Delete 、 modify
• Common relational operations ： choice 、 Projection 、 Connect 、 except 、 and 、 hand over 、 Bad 、 The cartesian product .
• Basic relationship operations (5 Kind of )： choice 、 Projection 、 and 、 Bad 、 The cartesian product .

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5. Classification of relational data languages

1. Relational algebra ： Express query requirements by operations on relationships .
2. Relationship calculus ： Using predicates to express query requirements .
3. Structured query language (SQL)： Between relational algebra and relational calculus .

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6. Outer code

set up F It's the basic relationship R One or a set of attributes of , But it's not the relationship R Code of ,K_s It's the basic relationship S The main code of . If F And K_s Corresponding , call F yes R The outer code of , said Basic relationship R For reference relations ( Relationship R Medium F Referring to the relationship S Medium K_s), Basic relationship S Is a referenced or target relationship .

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7. The integrity of the relationship

1. Entity integrity ： Main code ( In relation mode, the master code is used as the unique identifier ) The primary attribute in cannot be null .
2. Referential integrity ： Outer code or null value , Or equal to the main code value of a tuple in the referenced relationship .
3. User defined integrity ： The constraints that the application domain should follow .

Entity integrity and referential integrity are the integrity constraints that relational models must meet , It's called the two invariants of a relation .