Define foreign key. What is this concept used for?

Answer and Explanation:

The foreign key is a column that has been linked in a relational database such as mysql to a primary key in another table. We use the foreign key to show a relationship between 2 tables or more. Out of these two tables, one would have the primary key and another the foreign key. For these keys to be linked, any value that is in each of the foreign and primary keys must match.