Database Paradigm (1NF 2NF 3NF BCNF) detailed

The database design paradigm is the specification that database design needs to meet, the database that satisfies these specification is concise, the structure is clear, at the same time, does not take place insert (insert), delete and update operation exception. The reverse is a mess, not only to the database programmer to create trouble, and ugly, may have stored a large number of unnecessary redundant information.

Paradigm Description

1.1 The first paradigm (1NF ) columns with no Duplicates

The so-called First paradigm (1NF) refers to the fact that each column of a database table is an indivisible basic data item and cannot have multiple values in the same column, that is, an attribute in an entity cannot have multiple values or cannot have duplicate properties. If duplicate attributes are present, you may need to define a new entity, which is composed of duplicate attributes, and a one-to-many relationship between the new entity and the original entity. In the first normal form (1NF), each row of a table contains only one instance of information. In short, the first paradigm is a column with no duplicates.

Note: In any relational database, the first paradigm (1NF) is the basic requirement for relational schemas, and a database that does not meet the first normal form (1NF) is not a relational database.

For example, the following database tables are in accordance with the first paradigm:

Field 1

Field 2

Field 3

Field 4

Such database tables do not conform to the first paradigm:

Field 1	Field 2	Field 3	Field 4
		Field 3.1	Field 3.2

A field in a database table is a single attribute and cannot be divided. This single attribute consists of a basic type, including Integer, real, character, logical, date, and so on. Obviously, in any current relational database management system (DBMS), it is impossible for a fool to make a database that does not conform to the first paradigm, because these DBMS do not allow you to divide a column of a database table into two or more columns. Therefore, it is impossible for you to design a database that does not conform to the first paradigm in your existing DBMS.

1.2 The second paradigm (2NF ) property is completely dependent on the primary key [elimination of partial child function dependencies]

If the relationship pattern R is the first paradigm, and every non-primary property in R relies on a candidate key for R, then it is called a second-paradigm pattern.

The second paradigm (2NF) is established on the basis of the first paradigm (1NF), i.e. satisfying the second normal form (2NF) must first satisfy the first paradigm (1NF). The second normal form (2NF) requires that each instance or row in a database table must be divided by a unique region. It is often necessary to add a column to the table to store the unique identity of each instance. This unique attribute column is called the primary key or primary key, and the main code.

For example, the Employee Information table adds the employee number (emp_id) column because each employee's employee number is unique, so each employee can be uniquely differentiated.

In short, the second normal form (2NF) is completely dependent on the primary key for the non-master attribute.

The so-called complete dependency refers to the inability to exist only to rely on the main key part of the property (with function dependent w→a, if there is XW, there is x→a established, then called W→a is a local dependency, otherwise it is called w→a is a complete function dependency). If present, then this part of the attribute and primary key should be separated to form a new entity, and the new entity is a one-to-many relationship with the original entity.

Assume that the selection relationship table is Selectcourse (school number, name, age, course name, score, credits), keyword for the combination of keywords (study number, course name), because of the following decision relationship:

(School number, course name) → (name, age, score, credits)

This database table does not meet the second normal form because of the following decision relationship:

(course name) → (credits)

(school number) → (name, age)

That is, the presence of a field in the combo key determines the non-keyword situation.

Because it does not conform to 2NF, the following questions exist for this class selection relationship:

(1) Data redundancy:

The same course by N students elective, "credit" repeated n-1 times, the same student elective m courses, name and age repeated m-1 times.

(2) Update exception:

If the credit of a course is adjusted, the "credits" value of all the rows in the data sheet should be updated, otherwise the same course credit will be different.

(3) Insert exception:

Suppose a new course is to be opened and no one has yet been enrolled. Thus, the course name and credits cannot be recorded in the database because there is no "learning number" keyword.

(4) Delete exception:

Assuming that a group of students has completed elective courses, these elective records should be removed from the database table. At the same time, however, the course name and credit information were also removed. Obviously, this can also lead to an insertion exception.

Change the course of the elective selectcourse to the following three tables:

Student: Student (school number, name, age);

Course: Course (course name, credits);

Elective relationship: Selectcourse (School number, course name, score).

Such database tables conform to the second paradigm, eliminating data redundancy, update exceptions, insert exceptions, and delete exceptions.

In addition, all single-key database tables conform to the second normal form, as there is no possible combination of keywords.

1.3 The third paradigm (3NF ) property does not depend on other non-primary properties [elimination of transitive dependencies]

If the relationship pattern R is the second normal, and each non-primary attribute does not pass a candidate key that relies on R, then R is called a third-paradigm pattern.

Satisfying the third normal form (3NF) must first satisfy the second normal form (2NF). The third paradigm (3NF) requires that a database table not contain non-primary key information already contained in other tables.

For example, there is a departmental information table, where each department has a department number (dept_id), a department name, a department profile, and so on. Then the department number is listed in the Employee Information table, the department name, department profile and other departments related information can no longer be added to the Employee Information table. If there is no departmental information table, it should be built according to the third paradigm (3NF), otherwise there will be a lot of data redundancy.

The third paradigm (3NF): On the basis of the second paradigm, if there is no non-critical field in the data table the transfer function dependency on either of the candidate key fields conforms to the third paradigm. In short, the third paradigm is that properties do not depend on other non-principal properties.

The so-called transfer function dependency, refers to if there is a "a→b→c" decision relationship, the C transfer function depends on A.

Therefore, a database table that satisfies the third paradigm should not have the following dependencies:

key field → Non-critical field x→ non-critical field Y

Assume that the Student relationship table is student (school number, name, age, school, college location, college phone), the keyword is a single keyword "study number" because of the following decision relationship:

(school number) → (name, age, school, college location, college phone)

This database is 2NF compliant, but does not conform to 3NF because of the following decision relationship:

(school number) → (school) → (college location, college phone)

That is, there is a non-critical field "College location", "College phone" to the key field "study number" of the transfer function dependency.

It also has data redundancy, update exceptions, insert exceptions, and delete exceptions, which readers can analyze on their own.

The Student relations table is divided into the following two tables:

Student: (School number, name, age, school);

College: (College, location, telephone).

Such database tables conform to the third paradigm, eliminating data redundancy, update exceptions, insert exceptions, and delete exceptions.

1.4 Boyce- The canonical paradigm (BCNF is 3NF form of improvement)

If the relational mode R is the first paradigm, and each attribute does not pass a candidate key that relies on R. This relational pattern is the bcnf pattern. That is, on the basis of the third paradigm, if no field exists in the database table, the transfer function dependency of any candidate key field conforms to the Boyce-Christie paradigm.

Suppose the Warehouse Management Relationship table is storehousemanage (warehouse ID, store item ID, Administrator ID, number), and an administrator works only in one warehouse, and a warehouse can store multiple items. The following decision relationships exist in this database table:

(Warehouse ID, store item id) → (Administrator id, quantity)

(Admin ID, store item id) → (warehouse ID, quantity)

So, (warehouse ID, store item ID) and (Administrator ID, store item ID) are the candidate keywords for storehousemanage, the only non-critical field in the table is the number, which is in accordance with the third paradigm. However, the following decision relationship exists:

(warehouse id) → (Administrator id)

(Administrator id) → (warehouse id)

That is, the key field determines the critical field, so it does not conform to the BCNF paradigm. It will have the following exception:

(1) Delete exception:

When the repository is emptied, all "store item ID" and "quantity" information is deleted, and the "Warehouse ID" and "Administrator ID" information are also deleted.

(2) Insert exception:

You cannot assign an administrator to a warehouse when no items are stored in the warehouse.

(3) Update exception:

If the warehouse has been replaced by an administrator, the administrator ID for all rows in the table is modified.

Break down the Warehouse management relationship table into two relational tables:

Warehouse Management: Storehousemanage (warehouse ID, administrator ID);

Warehouse: Storehouse (warehouse ID, store item ID, quantity).

Such database tables conform to the BCNF paradigm, eliminating deletion exceptions, insert exceptions, and update exceptions.

The following relationships exist between the four paradigms:

Reference: http://jacki6.iteye.com/blog/774866