What is Foreign keys​ in database

Foreign keys are used in database to reference unique columns in another table.
Foreign keys connected with the concept of referential integrity.

What is Structured Query Language(SQL)

SQL is Structured Query Language. e.g.MySQL and Microsoft SQL Server both are relational database management systems)
Data Definition L (DDL) – Create, Alter, Drop
Data Manipulation Language (DML)- Select , Insert, Update, Delete
Data Control Language(DCL) – Grant, Revoke