What is the difference between primary key and foreign key in mysql

The difference between primary key and foreign key: Function: The primary key uniquely identifies the record, and the foreign key associates records in different tables. Uniqueness: The primary key is unique and no duplication is allowed; the foreign key is not unique and can be repeated. NULL values: NULL values ​​are not allowed for primary keys and NULL values ​​are allowed for foreign keys. Relationship: The primary key is the relationship within the table, and the foreign key is the relationship between tables. Constraints: The primary key has non-null and unique constraints, and the foreign key has constraints that reference the primary key of the main table. Quantity: Each table can only have one primary key, and a table can have multiple foreign keys.

The difference between primary key and foreign key in MySQL

Primary key

• A set of columns that uniquely identify each record in each table
• Cannot be empty (non-null constraint)
• And cannot be repeated (unique constraint)
• In MySQL, each table can only have one primary key

Foreign key

• A column or column corresponding to the primary key in another table Group column
• The foreign key column must refer to the primary key that exists in the main table
• The foreign key column can be empty (null values ​​are allowed)
• In MySQL, a table can have multiple Foreign keys

Specific differences

Example

• Primary key example: A table named "Users" whose primary key is the "user_id" column. Each record has a unique "user_id" that can be used to identify the user.
• Foreign key example: A table named "Orders" whose foreign key is the "user_id" column. This key references the "user_id" primary key in the "Users" table, indicating that each order is associated with a specific user.

Function

• Primary key: Ensure the uniqueness of records in the table and optimize the data retrieval speed.
• Foreign keys: Establish relationships between tables, maintain data integrity and prevent orphan records.

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