What is the difference between sql primary key and foreign key

The difference between sql primary key and foreign key: 1. The primary key uniquely identifies a record, cannot be repeated, and is not allowed to be empty, while the foreign key can be repeated and can be a null value; 2. The primary key is used to maintain Data integrity, foreign keys are used to establish connections with other tables; 3. There is only one primary key, but there can be multiple foreign keys.

The difference between sql primary key and foreign key: there is only one primary key, which cannot be repeated and no null value is allowed. However, there can be multiple foreign keys, which can be repeated or not. There are null values; the primary key is used to maintain data integrity, and the foreign key is used to establish connections with other tables

What are primary keys and foreign keys

There are several attributes in a record in a relational database. If a certain attribute group (note the group) can uniquely identify a record, the attribute group can become a primary key
For example

Student table (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

Grade table (student number, course number, grade)

Grade table A single attribute cannot uniquely identify a record. Only the combination of student number and course number can uniquely identify a record. Therefore, the attribute group of student number and course number is a primary key.
The student number in the score table is not the primary key of the score 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, then the student number in the grade table is said to be the foreign key of the student table

Same as the grade table The course number in is the foreign key of the course schedule

The primary key and foreign key are defined mainly to maintain the integrity of the relational database. To summarize:

1. The primary key is the only key that can determine a record. Identification, for example, a record includes identity number, name, and age.

The ID number is the only one that can identify you. Others may be duplicated. Therefore, the ID number is the primary key.

2. Foreign keys are used to associate with another table. It is a field that can determine the record of another table and is used to maintain data consistency.

For example, if a field in table A is the primary key of table B, then 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, cannot be repeated, and is not allowed to be empty; while foreign keys can be repeated and can Is a null value

(2) The primary key is used to maintain data integrity, and the foreign key is used to establish connections with other tables

(3) There is only one primary key, and foreign keys can have Multiple

The above is the detailed content of What is the difference between sql primary key and foreign key. For more information, please follow other related articles on the PHP Chinese website!