Foreign key is a type of attribute or table in a relational database that is used to identify a specific row of data within another table. It is used to link two tables together by referencing a particular column or row within a table. Using a foreign key can not only help to quickly query data but also foster data integrity and integrity constraints by preventing data loading errors.
Foreign keys are typically placed in a separate table in a one-to-many relationship, with each row in the source table representing a unique value in the foreign key table. This can help to maintain the referential integrity of the data by enabling the query to read and process the entries in the two related tables simultaneously, rather than having to store and manage each entry independently.
Foreign keys are typically declared by specifying a column of the source table as a unique identifier when creating the foreign key table. This can either refer to a separate column, or a combination of columns and/or their values, which uniquely identifies each row in the source table. Once an entry is added to the foreign key table, it must be included in the source table’s primary key to ensure data integrity.
Foreign keys can be combined with other table constraints to ensure the integrity of the data and to ensure that data entry is not affected by any manipulation of the source table. For example, a foreign key constraint may be used to prevent deleting a data row in the source table if it is referenced in the foreign key table. Additionally, a foreign key can be used in conjunction with other table constraints, such as a check constraint, to ensure that all data stored in the foreign key table matches the data in the source table.
Foreign keys are important in keeping data integrity in relational databases, ensuring that data is consistent and that the impact on data integrity is minimal when manipulating data within a database. They are also a means of enforcing the deletion of records in the source table if an associated foreign key table does not exist. They are necessary components of many database architectures, and can be used to ensure data accuracy in processing operations.