What is a Foreign Key

A foreign key is a column or set of columns in a database table that establishes a relationship between two tables. It creates a link between the data in two tables by referencing the primary key or a unique key of another table. Here are some key points about foreign keys:- 

1. Referential Integrity:-  The primary purpose of a foreign key is to enforce referential integrity between related tables in a relational database. It ensures that the values in the foreign key column(s) of one table correspond to the values in the primary key or unique key column(s) of another table.

2. Relationships:-  Foreign keys define relationships between tables in a database. The table containing the foreign key is called the child table, and the table containing the referenced primary key or unique key is called the parent table. A foreign key column in the child table refers to the primary key or unique key column(s) in the parent table.

3. Constraint:-  A foreign key is also a type of constraint that ensures data consistency and integrity. It prevents actions that would violate the relationship between tables, such as inserting rows with non-existent foreign key values or deleting rows that have dependent child rows.

4. Cascade Actions:-  In some cases, foreign key constraints can specify actions to be taken when a related row in the parent table is updated or deleted. These actions include cascading updates and cascading deletes, which automatically propagate changes to related rows in the child table.

5. Syntax:-  Foreign keys are typically defined when creating or altering a table using SQL. The syntax for defining a foreign key constraint includes specifying the foreign key column(s) and referencing the parent table's primary key or unique key column(s).

For example, in a database with tables for Orders and Customers, the Orders table might contain a foreign key column called CustomerID that references the CustomerID primary key column in the Customers table. This foreign key relationship ensures that every order is associated with a valid customer, maintaining data integrity and consistency.