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This article mainly introduces the differences between sql primary keys and foreign keys, which can be used for reference by friends who need them. I hope you will learn a lot after reading this article. Next, let the editor take you to learn about it.

The difference between sql primary key and foreign key: there is only one primary key, which cannot be repeated or allowed to have null values, while foreign keys can have multiple, repeatable or null values; primary keys are used to maintain data integrity, and foreign keys are used to establish relationships with other tables.

What are primary keys and foreign keys?

There are several attributes in a record in a relational database. If one of the attribute groups (note that the group) can uniquely identify a record, the attribute group can become a primary key.

such as

Student form (student number, name, gender, class)

The student number of each student is unique, and the student number is a primary key.

Course schedule (course number, course name, credits)

The course number is unique, and the course number is a primary key.

Transcript (student number, course number, grade)

A single attribute in the score sheet cannot uniquely identify a record, and only the combination of the student number and the course number can uniquely identify a record, so the attribute group of the student number and the course number is a primary key.

The student number in the grade table is not the primary key of the grade table, but it corresponds to the student number in the student table, and the student number in the student table is the primary key of the student table, so it is said that the student number in the grade table is the foreign key of the student table.

Similarly, the course number in the score sheet is the foreign key of the course schedule.

The main purpose of defining primary and foreign keys is to maintain the integrity of the relational database. To sum up:

1. A primary key is a unique identification that identifies a record, for example, a record includes an identity plus sign, name, and age.

The ID number is the only one that can identify you, and everything else may be duplicated, so the ID number is the primary key.

two。 Foreign keys are used to associate with another table. Is a field that determines the record of another table and is used to maintain data consistency.

For example, if a field in table An is the primary key of table B, it can be the foreign key of table A.

The difference between primary key and foreign key

(1) the primary key uniquely identifies a record, which cannot be duplicated and is not allowed to be empty, while the foreign key can be repeated and can be null.

(2) Primary keys are used to maintain data integrity, and foreign keys are used to establish connections with other tables.

(3) there is only one primary key and multiple foreign keys.

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