Judge the difference between the main code and the post selected code, and the main code and the can

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What does the main code and the subsequent code selection mean?

Be clear.

A master keyword, or a primary keyword, is one or more fields in a table, and its value is used to uniquely identify a record in the table. In a relationship between two tables, the primary keyword is used in one table to refer to a specific record from the other table.

A primary keyword is a unique keyword, and a table defines a portion of God's command. The primary key of a table can be composed of multiple keywords, and the column of the primary keyword cannot contain null values. The main keyword is chosen by Kejia and can be defined in a create table or alter table statement.

Example of master code:

Signing a contract (actor's name, production company, movie name).

External code: There are two relationships R and S, X is an attribute or attribute group of R, and X is not the code of R, but X is the code of S (or has the same meaning as the code of S), then X is the foreign key of R, referred to as the foreign code or foreign key.

Such as: employees (employee number, name, gender, title, department number).

Department (department number, department name, **, person in charge).

Among them, the "department number" in the employee relationship is a foreign code of the employee relationship.

It should be noted here that when the definition says that x is not a code of r, it does not mean that x is not a primary attribute of r, x is not a code, but it can be a constituent attribute of the code, or a primary attribute of any candidate code.

The main code is uniquely identified, and the candidate code is the code that can be used as the main code, and the main code must be a subset of the candidate code, but the candidate code is not necessarily the main code.

Master code: the candidate code selected by the database designer Kuanzhen to distinguish different entities in the same entity set; In addition, attributes that never or rarely change should be chosen;

Candidate code: If the true subset of any supercode cannot include the supercode, it is called a candidate code; Supercodes include candidate codes;

All codes are a collection, and all collections that can be used to identify unique entities in an entity set are supercodes. If the true subset of any supercode cannot include a supercode, it is said to be a candidate. A candidate selected by the database designer to distinguish between different entities in the same entity set.

The supercode includes the candidate code, and the candidate code includes the main code.

This distinction is the same as the difference between players on the field and substitute players, and the candidate code is useful if there is a problem with the main code.

Referenced from Miao Xuelan's "Database System Principles and Application Tutorial" P24-25 code is also known as a keyword, which can be the only one.

Identify an entity. Appearance.

A code can be an attribute or a property group, but the property group cannot contain redundant attributes.

When there are more than one entity in a setyards, select one of the codes asMaster code,The rest of the codes areCandidate code.

Added: Entity sets cannot be displaced to represent the bid code of the entity attribute. A primary code value corresponds to one instance, while a secondary code value corresponds to multiple instances.

Master code and bai

Candidate codes are used to uniquely identify relationships.

There can be multiple candidate dao in a relationship

Just pick one of them back as the master.

Answer: The attributes contained in the main code are called primary attributes.

Here's an example. In the student table:

Attributes: Student ID, Name, Gender, Age, Class, Department.

Generally, the student number can uniquely identify the identity of a classmate, and we can set the student number as the main code. is the simplest candidate code.

Then, of course, student number + name can also be used as a unique identifier, and can also be used as a candidate code, in the same way, student number + name + gender can also be used as a candidate code.

And so on. The most extreme case is that the whole table is used as the main code, and the main code at this time is also called the full code.

The primary code is the attribute or smallest set of attributes used in a relationship to uniquely determine a tuple. Sometimes, there may be multiple attributes that can be used as the main code in a relationship, then these attributes are called candidate code attributes, and the corresponding codes are called candidate codes. The primary code is an attribute that is selected from the candidate code as the main code.

If the value of an attribute group in a relationship can uniquely identify a tuple, the attribute group is called a candidate stove wheel mountain code.

If there are multiple candidate codes for a relationship, any one of them can be used as the main code, and the attributes of the main code are called the main attributes.

If there is a field in a table and the primary key of the other table is stored, this field is a foreign key, also known as a foreign code. In the relationship pattern A (city, street, zip code), only by knowing the city and the street or the street and the zip code can an address be uniquely determined, in the two tuples, the city name, the street name, and the postal code may be the same, and neither of them can uniquely identify the tuple. References:

Introduction to Database Systems (21st Century Curriculum Textbook) 3rd Edition, Sa Shixuan.

Edited by Wang Shan.