A foreign key is a column or a set of columns in one database table that refers to the primary key in another table, establishing a link between the two tables. This relationship enforces referential integrity, ensuring that the value in the foreign key column must exist in the referenced primary key column of the other table or be NULL

. In practical terms, a foreign key creates a "parent-child" relationship between tables. For example, in an orders table, a foreign key column like customer_id might reference the id column in a customers table, ensuring every order is associated with a valid customer

. The foreign key constraint prevents invalid data entry by disallowing values in the foreign key column that do not exist in the referenced primary key column. This maintains data consistency and prevents orphan records-rows in the child table without corresponding rows in the parent table

. In summary, a foreign key:

• Is a field or set of fields in one table that references the primary key of another table.
• Enforces referential integrity between the two tables.
• Ensures that values in the foreign key column exist in the referenced primary key column.
• Helps maintain consistent and accurate relationships in relational databases