The relationships managed by relational database management systems are

The relationships managed by the relational database management system are several two-dimensional tables. Because in a relational database, a table is a relationship, and the relationships managed by the relational database management system are multiple two-dimensional tables.

Relational database is a database based on the relational database model. It uses concepts and methods such as set algebra to process the data in the database. It is also a data that is organized into a set of formally descriptive tables. The essence of this form is to load a special collection of data items. The data in these tables can be accessed or retrieved in many different ways. Convene without reorganizing database tables. (Recommended learning: MySQL Video Tutorial)

The definition of a relational database 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 the standard user and application interface to the 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. Mainstream relational databases include oracle, db2, sqlserver, sybase, mysql, etc.

Relationship model structure

1. Single data structure----relationship (table file). The table of a relational database uses a two-dimensional table to store data. It is a logical group with related information arranged in rows and columns. It is similar to an Excel worksheet. A database can contain any number of data tables.

From the user's perspective, the logical structure of a relational model is a two-dimensional table, consisting of rows and columns. This two-dimensional table is called a relationship. In layman's terms, a relationship corresponds to a table.

2. Tuple (record). A row in the table is a tuple, or a record.

3. Attributes (fields). Each column in the data table is called a field. The table is defined by the various fields it contains. Each field describes the meaning of the data it contains. The design of the data table is actually the design of the fields. When creating a data table, each field is assigned a data type, defining their data length and other properties. Fields can contain various characters, numbers, and even graphics. Such as error! Reference source not found.

4. Attribute value. The intersection of rows and columns represents a certain attribute value. For example, "Database Principles" is the attribute value of the course name

5 and the main code. The primary key (also called the primary key or primary key) is the data used to uniquely identify a tuple in the table. Keywords are used to ensure the uniqueness of records in a table. They can be one field or multiple fields and are often used as index fields of a table. The key of each record is different, so it can uniquely identify a record. The key is also called the primary key, or simply the primary key. Such as error! Reference source not found.

6. Domain. The value range of the attribute.

7. Relationship model. The description of relationships is called a relationship schema. The description of the relationship is generally expressed as: relationship name (attribute 1, attribute 2...attribute n). For example, the above relationship can be described as: course (course number, course name, credits, teacher).

But this simple data structure of the relational model can express rich semantics and describe entities in the real world and various relationships between entities.

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