Soft test - database (2) relational model

1.1 relational model

1.1.1 Definition  

        From the user's point of view , The logical structure of data in a relational model is a two-dimensional table , It consists of rows and columns ;

         Express entity set with table structure , Use foreign keys （ Outer code ） Indicates the entity simple connection ;

1.1.2 Advantages and disadvantages

        advantage ：

        Based on strict mathematical concepts ;

        A single concept , Simple structure 、 Clear 、 It's easy for users to understand and use ;

        Access path is transparent to users , So data independence 、 Good safety , Simplify database development ;

        

        shortcoming ：

        Because the access path is transparent , Query efficiency is often lower than that of non relational data model ;

1.2 Basic concepts

1.2.1 Domain

        Is a set of values with the same data type ;

1.2.2 The cartesian product

        Two sets X and Y The Cartesian product of , Also called direct product , Expressed as X * Y , The first object is X And the second object is Y One of the members of all possible ordered pairs .

        example ：X = { a ,b ,c },Y = { 1 , 2 , 3 }

        X * Y = {

                      < a ,1 >,< a ,2 >,< a ,3 >,

                      < b ,1 >,< b ,2 >,< b ,3 >,

                      < c ,1 >,< c ,2 >,< c ,3 >, 

                     }

 1.2.3 The expression of relationship

        Express ：

        D1 * D2 * …… * Dn A subset of is called in the domain D1,D2,……,Dn Relationship on , Expressed as

                R（ D1,D2,……,Dn ） 

        R： Relationship name ;

        n： The purpose or degree of a relationship ;

        

        Relationships are also a two-dimensional table , Each row of the table corresponds to a tuple , Each column of the table corresponds to a field ;

        

        attribute ：

        Different columns in a relationship can correspond to the same domain

        To make a difference , You have to give each column a name , Called attribute ;

        n There must be a relationship between eyes n Attributes ;

        

        Candidate code ：

        If the value of an attribute group in a relationship can uniquely identify a tuple , The attribute group is called candidate code ;

        

        Main code ：

        If a relation has more than one candidate code , Select one of the main codes ; 

 1.2.4 The nature of the basic relationship

        The basic relationship has the following 6 Article nature ：

        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 field , Call each of these columns an attribute , Different attributes should be given different attribute names ;

        3. The order of the columns doesn't matter , That is, the order of columns can be exchanged arbitrarily ;

        4. Any two tuples cannot be exactly the same ;（ But in most practical relational database products , for example Oracle , If the user does not define relevant constraints , They both allow two identical tuples in the relational table  ） 

        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 be an indivisible data item ;

 1.3 normal form

1.3.1 First normal form

        If the relationship pattern R Every relationship r Are inseparable atomic values , So called R It is the mode of the first paradigm ,r Is a normalized relationship ;

        In any relational database , The first paradigm is the basic requirement of relational model , Not satisfied with the first paradigm （ 1NF ） Is not a relational database ;

 1.3.2 Second normal form

        If the relational pattern R yes 1NF, And each non primary attribute is completely functionally dependent on candidate keys , So called R yes 2NF Pattern ;

1.3.3 Third normal form

         If the relationship pattern R yes 1NF , And each non primary attribute does not pass dependent on R The candidate code for , said R yes 3NF ; 

1.3.4 BC normal form  

        If the relational pattern R yes 1NF , And each attribute is not passed since R Candidate key for , So called R yes BCNF Pattern ;