Introduction to three design paradigms of relational databases

Today, a business requirement involves the third paradigm.

 

Sort out the concepts here to facilitate future queries:

 

1. 1nf)

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.

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. For example, for the employee information table in Figure 3-2, the employee information cannot be displayed in one column or two or more columns in one column; each row in the employee information table only represents the information of one employee. The information of one employee appears only once in the table. In short, the first paradigm is a non-repeated column.

2 second Paradigm (2nf)

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. 3-2 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 non-primary attributes are not partially dependent on primary keywords.

3. Third Paradigm (3nf)

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 listing the Department numbers in the employee information table in Figure 3-2, 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.