A foreign key in a My Sql database is a field (or fields) of a table that points to a primary key of an other table. Such keys are defined in a database to ensure that only fields that supposed to appear in a database may be permitted.
What is their requirement ? Why are they useful ?
For enhancing the referential integrity of your database, you need foreign key. Read more on how they are defined and created. Learn through some examples.
Foreign keys and Referential Integrity in database « My Technology Blog
Foreign keys in MySQL and referential integrity
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