The main difference between Primary key and Foreign key in a table is that it’s the same column which behaves as primary key in the parent table and as a foreign key in a child table. For example in Customer and Order relationship, customer_id is the primary key in Customer table but a foreign key in Order table. By the way, what is a foreign key in a table and difference between Primary and Foreign key are some of the popular SQL interview questions, much like truncate vs delete in SQL or difference between correlated and noncorrelated subquery? We have been learning key SQL concepts along with these frequently asked SQL questions and in this SQL tutorial, we will discuss what is a foreign key in SQL and purpose of the foreign key in any table. By the way, this is the third article related to a primary key in SQL, other being difference between primary and unique key and How to find second highest salary in SQL. If you are preparing for any technical job interview where you expect some SQL questions, check out these questions, they are worth preparing.
What is Foreign key in a table
What is foreign key and difference between primary vs foreign key in SQLA Foreign key is a column in one table which is the primary key on another table. Foreign key and a Primary key is used to define relationship between two tables in relational database. For example in Employee and Department relationship, we have two tables Department(dept_id, dept_name) and  Employee (emp_id, emp_name, dept_id). dept_id is primary key in Department table and foreign key in Employee table. 

Though it’s not require that name of foreign key must be same with primary key, we have kept it same as per standard SQL best practices. Foreign key in a table enforce Referential Integrity constraint, which can be used to implement business rules e.g. referential integrity can stop you from creating an Employee with a non existent department. 

Here is some important difference between primary and foreign keys in a table which is worth remembering both on SQL interview point of view and knowledge point of view.

1) Name of foreign key can be different than the name of primary key it represent in other table. For example in our Employee and Department relationship, Primary key in Department table is dept_id  and we have used same name in Employee table to create foreign key. It could have been different e.g. departmentId or departmentID t etc.

2) Another difference between primary and foreign key is that unlike primary key, a foreign key can be null e.g. in our example you can have an Employee record for which dept_id can be null, this shows that no corresponding record in Department table.

3) One more difference between primary key and foreign key is that foreign key can be duplicate opposite to primary key which is always unique.

4) By using foreign key constraints, we can introduce referential integrity in multiple table relationship in SQL. Referential integrity guarantees data integrity, see benefits of Referential Integrity in SQL to know more.

5) Foreign key mostly works as a link between two table when we join tables using INNER JOIN and OUTER JOIN. For example, when we INNER JOIN both Employee with Department table, we can use dept_id as joining column. See How to join three tables in SQL for more details.

6) Table on which a column is declared as a primary key is known as parent table in the relationship and foreign key table is known as child table in a relationship. For example in Employee and Department relationship, Department is parent table because dept_id is primary key there and Employee is child table because dept_id is a foreign key in this table.

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