A foreign key is a key field (column) in a database table, which relates the table to another table where the key is a primary key. The primary - foreign key relations are used to cross-reference database tables.
The foreign key is a SQL locking mechanism.
The foreign key is a column that can have NULL values.
Locking is the process of database authentication.
Locking is a process preventing users from reading data being changed by other users, and prevents concurrent users from changing the same data at the same time.