Database paradigm Review

Introduction

The relationships in relational databases must meet certain requirements, that is, they must meet different paradigms.

Currently, relational databases have six paradigms: 1nf, 2nf, 3nf, 4nf, and 5nf) and the sixth paradigm (6nf ). The first paradigm (1nf) meets the minimum requirements ). The second Paradigm (2nf) that meets more requirements on the basis of the first paradigm, and the other paradigms are similar. Generally, databases only need to satisfy the 3nf.

No repeated column in the first paradigm (1nf)

The first paradigm (1nf) means that each column in the database table is an inseparable basic data item. The same Column cannot contain multiple values, that is, an attribute in an object cannot have multiple values or duplicate attributes. If duplicate attributes exist, you may need to define a new object. A new object consists of duplicate attributes. The new object has one-to-multiple relationships with the original object. In the first paradigm (1nf), each row of the table contains only information of one instance. In short, the first paradigm is a non-repeated column.

Note: In any relational database, the first paradigm (1nf) is the basic requirement for the relational model. databases that do not meet the first paradigm (1nf) are not relational databases.

2nf attribute

Completely dependent on the primary key [removing some function dependencies of non-primary attributes on the primary code]

The second Paradigm (2nf) is established on the basis of the first paradigm (1nf), that is, to satisfy the second Paradigm (2nf) must satisfy the first paradigm (1nf) first ). The second Paradigm (2nf) requires that each instance or row in the database table be able to be distinguished by a unique region. To implement differentiation, you usually need to add a column to the table to store the unique identifier of each instance. For example, the employee ID (emp_id) column is added to the employee information table. Because each employee's employee ID is unique, each employee can be uniquely distinguished. This unique attribute column is called as the primary keyword, primary key, and primary code.

The second Paradigm (2nf) requires that the attributes of an object fully depend on the primary keyword. The so-called full dependency refers to the fact that there cannot be an attribute that only depends on a part of the primary keyword. If so, this attribute and this part of the primary keyword should be separated to form a new entity, the relationship between the new object and the original object is one-to-multiple. To implement differentiation, you usually need to add a column to the table to store the unique identifier of each instance. In short, the second paradigm is that attributes fully depend on the primary key.

3nf attributes

Independent from other non-primary attributes [Removing transfer dependencies]

The third paradigm (3nf) must satisfy the second Paradigm (2nf) first ). In short, the third paradigm (3nf) requires that a database table do not contain information about non-primary keywords already contained in other tables. For example, there is a department information table, where each department has a department ID (dept_id), department name, Department profile, and other information. After the Department numbers are listed in the employee information table, you cannot add the Department name, Department profile, and other information related to the department to the employee information table. If the department information table does not exist, it should also be constructed based on the third paradigm (3nf), otherwise there will be a large amount of data redundancy. In short, the third paradigm is that attributes do not depend on other non-primary attributes.