what is foreign key

A foreign key is a column or combination of columns in a database table that is used to establish a link between two tables. It refers to the primary key of another table and helps define the relationships between tables in a relational database. Foreign keys allow developers to maintain referential integrity across their database, prevent errors, and improve the performance of any operation. They also promote consistency by making it impossible to add a row in a table with a foreign key that doesn't exist in the referenced table. Foreign keys are not mandatory, and a table may have no foreign keys, but they are useful in relational databases to streamline data sets and promote efficiency.