How does a relational database realize the connection between data?

The method for realizing the connection between data in relational database is public attributes; the connection between data is realized by one table containing the primary key of another table. The foreign key is used to establish the connection between the data of two tables. Attributes, foreign keys can be composed of one or more attributes, and the foreign key value reference is referenced to the value of the primary key, establishing a relationship between the two data.

The operating environment of this tutorial: Windows 10 system, DELL G3 computer.

What is the method for realizing the connection between data in a relational database?

In a relational database, the connection between data is realized by including the primary key (public attribute) of another table in one table.

Foreign keys are attributes used to establish a relationship between two table data. Foreign keys can be composed of one or more attributes. The foreign key value refers to the value of the primary key of the referenced table, establishing a relationship between the two data

Relational database

Relational database, It is a database built on the basis of the relational database model. It uses concepts and methods such as set algebra to process data in the database. It is also organized into a set of formally descriptive tables. The essence of this form of tables is to load A special collection of data items. The data in these tables can be accessed or recalled in many different ways without the need to reorganize the database tables. A relational database definition results in a table of metadata or a formal description of tables, columns, ranges, and constraints. Each table (sometimes called a relationship) contains one or more data types represented by columns. Each row contains a unique data entity of the type defined by the column. When creating a relational database, you can define the range of possible values ​​for a data column and further constraints that may apply to that data value. The SQL language is a standard user and application interface to a relational database. The advantage is that it is easy to expand, and after the initial database creation, a new data type can be added without modifying all existing applications.

Concept

## Relational database is the mainstream of database applications, and many database management systems Data models are developed based on relational data models.

1) Relational database: In a given application field, the collection of all entities and the relationships between entities constitutes a relational database.

2) Type and value of relational database: The type of relational database is called relational database schema, which is a description of the relational database, the definition of several fields, and several relational schemas defined on these fields; A value is a collection of relationships corresponding to these relational schemas at a certain moment, often referred to as a relational database.

Relational databases are divided into two categories: one is desktop database, such as Access, FoxPro and dBase, etc.; the other is client/server database, such as SQL Server, Oracle, Sybase, etc. Generally speaking, desktop databases are used for small, stand-alone applications. They do not require a network or server and are more convenient to implement, but they only provide data access functions. Client/server database is mainly suitable for large-scale, multi-user database management systems. The application program consists of two parts: one part resides on the client and is used to display information to users and interact with users; the other part resides on the server , mainly used to implement database operations and data calculation processing.

Expand knowledge

The relational model uses a two-dimensional table to represent the entity set, and uses public attributes to realize the connection between entities. A relational database consists of several tables. The data between tables is connected by including the primary key (common attribute) of another table in one table.

1. Relationship

Relation is a two-dimensional table in which rows and columns intersect. A row in the table is called a tuple of the relationship, and a column in the table is called an attribute of the relationship. Each attribute has a value range, called the attribute domain. A collection of tuples is called a relationship instance, and usually a relationship instance is simply called a relationship.

2. The nature of the relationship

The relationship is a standardized two-dimensional table. In the relational model, the relationship must satisfy the following properties:

• All data in each column are of the same type and come from the same domain

• Each column has a unique column name.

• The order of the columns in the table does not matter

• No two rows in the table can be exactly the same

• The order of the rows in the table does not matter either

• The intersection of rows and columns must be a single value (not another set of values)

3. Data type

When defining the table structure, the data type of the column must be determined. Most database systems support at least the following data types

• Number: data that can be used for arithmetic operations

• Character: character data Also called a string, it is a text string composed of any characters or symbols. For example, name, address, and major are all character data

• Date: Date data is stored in a specific format, and special mathematical operations can be performed on date data. For example, subtract one date from another to find the number of days between two dates.

• Logic: Logical data can only have two values: true or false. For example, there are only two states of marriage, all of which can be defined as logical data

4. The key

is in one No two tuples in a relation are allowed to have the same value for a key. Conceptually, keys can be subdivided into super keys, candidate keys, primary keys and foreign keys

Super keys

Any attribute or attribute that can uniquely identify a tuple Groups are called superkeys

Candidate keys

The candidate keys are the smallest superkeys, and any proper subset of them cannot become superkeys

Primary key

There may be multiple candidate keys in a relationship, select one of the candidate keys as the primary key

Foreign key

In a relational database, the connection between data is achieved by including the primary key (public attribute) of another table in one table.

Foreign key is an attribute used to establish a connection between the data of two tables. , a foreign key can consist of one or more attributes. The foreign key value refers to the value of the primary key of the referenced table, establishing a connection between the two data

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