MySQL Foreign Key Constraints

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A foreign key is a type of database constraint used in relational databases such as MySQL. It is used to ensure data integrity by defining a relationship between two tables. A foreign key is a column or set of columns in one table that refers to the primary key of another table.

 

When creating a foreign key, the data in the foreign key column must match the data in the primary key column of the referenced table. This is to ensure that the data in the foreign key column is valid. For example, if a customer table has a foreign key to an address table, the customer's address must match an address in the address table.

 

A foreign key can also be used to enforce referential integrity. Referential integrity ensures that when data is inserted, updated, or deleted in one table, the related data in the other table is also affected. For example, if a customer is deleted from the customer table, all of the orders associated with that customer should also be deleted.

 

Foreign keys can also be used to create relationships between tables, such as one-to-one, one-to-many, and many-to-many. A one-to-one relationship is when one row in a table is related to one row in another table. A one-to-many relationship is when one row in a table is related to many rows in another table. A many-to-many relationship is when many rows in a table are related to many rows in another table.

 

In summary, a foreign key is a type of database constraint used in relational databases such as MySQL. It is used to ensure data integrity by defining a relationship between two tables. It can also be used to enforce referential integrity and create relationships between tables.