SQL FOREIGN KEY In the relational databases, a foreign key is a field or a column that is used to establish a link between two tables.
In simple words you can say that, a foreign key in one table used to point primary key in another table.
A foreign key is a column (or columns) that references a column (most often the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words, only values that are supposed to appear in the database are permitted.
For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders. Business logic requires that all orders must be associated with a customer that is already in the CUSTOMER table. To enforce this logic, we place a foreign key on the ORDERS table and have it reference the primary key of the CUSTOMER table. This way, we can ensure that all orders in the ORDERS table are related to a customer in the CUSTOMER table. In other words, the ORDERS table cannot contain information on a customer that is not in the CUSTOMER table.
Difference between primary key and foreign key in SQL:
These are some important difference between primary key and foreign key in SQL-
Primary key cannot be null on the other hand foreign key can be null.
Primary key is always unique while foreign key can be duplicated.
Primary key uniquely identify a record in a table while foreign key is a field in a table that is primary key in another table.
There is only one primary key in the table on the other hand we can have more than one foreign key in the table.
By default primary key adds a clustered index on the other hand foreign key does not automatically create an index, clustered or non-clustered. You must manually create an index for foreign key.