Three relationships between relational database tables and tables, three types of databases

Three relationships between relational database tables and tables, three types of databases
1. One-to-one relationship: Definition: There are two tables. a row in the first table is only related to a row in the second table, and a row in the second table, it is also related to only one row in the first table. We call these two tables a one-to-one relationship. For example, the first table:

ID	Name	Nationality	Contribution
1001	Wang Damai	China	Never imagined
1002	Jizemingbu	Japan	Action Art Film
1003	Blade queen	Unknown	Family planning

The second table:

ID	Date of birth	Date of death
1001	1988	NULL
1002	1984	NULL
1003	Unknown	3XXX

Each row in the first table corresponds to only one row in the second table. Tip: one-to-one relationships are rare. However, this type is still required in some cases. Case 1: A table contains too many data columns. Case 2: Separate data into different tables and divide the data into different security levels. Case 3: extract common data columns to form a table 2, one-to-many relationship: Definition: there are multiple tables, the rows in the first table can be associated with one or more rows in the second table, but the rows in the second table can only be associated with the rows in the first table. For example, the first table:

ID	Name	Nationality
1001	Wang Damai	China
1002	Jizemingbu	Japan
1003	Blade queen	Unknown

The second table:

ID	Program ID
1001	10001
1001	10002
1003	10003
1003	10004

The third table:

Program ID	Program name
10001	Never imagined
10002	Report to the boss
10003	Interstellar 2
10004	League of legends

The row in the first table can correspond to multiple rows in the second table; the row in the second table corresponds to the row in the third table. Tip: One-to-multiple relationships are the most common link types. 3. Multi-to-Multi-Relationship: Define: There are two tables. The row of the first table can be associated with one or more rows in the second table. At the same time, a row in the second table can be associated with one or more rows in the first table. For example, the first table:

ID	Name	Nationality
1001	Wang Damai	China
1002	Jizemingbu	Japan
1003	Blade queen	Unknown

The second table:

ID	Program ID
1001	10001
1001	10002
1003	10003
1003	10004

The third table:

Program ID	Program name
10001	Never imagined
10002	Report to the boss
10003	Interstellar 2
10004	League of legends

Between the first and third tables, one person can participate in multiple programs, or multiple people can participate in one program. For example, Mr. Wang participated in "never imagined" and "Report to the boss". Wang Damai and Ji zemingbu are in "never imagined. This is a many-to-many relationship.