Difference between Primary Key and Foreign Key in Sql Server

Introduction: In this article I am going to share one of the mostly asked Sql interview question i.e. Primary Key vs Foreign Key. 

In previous related articles I explained How to Create, check and drop Foreign Key Constraint on Sql server table  and Create table in sql server with auto increment primary key column and Difference between Temporary Table and Table Variable in Sql Server and 20 main differences between Stored procedures and Functions in Sql Server and Copy all data from one database table to another database table in Sql Server 

I have tried to list all the major differences between Primary Key and Foreign Key. Hope it will help you in preparing for your interview.

Primary Key	Foreign Key
A primary key is a column or a set of columns that can be used to uniquely identify a row in a table. It enforces the implicit NOT NULL constraint.	A foreign key is a column(s) in one table that references a unique column usually the primary key of another table. The purpose of the foreign key is to ensure referential integrity of the data. The referenced table is called the parent table while the table with the foreign key is called the child table.
Syntax to create Primary Key: CREATE TABLE tbDepartment  ( --define primary key   DeptId INT IDENTITY(1,1) PRIMARY KEY,    DeptName varchar (50) NOT NULL  )	Syntax to create foreign key: CREATE  TABLE tbEmployee  ( --define primary key     EmpId INT IDENTITY(1,1) PRIMARY KEY,      FirstName VARCHAR (50) NOT NULL,     LastName VARCHAR (50) NOT NULL,          --define foreign key     DeptId INT FOREIGN KEY REFERENCES tbDepartment(DeptId)  )
Primary Key doesn’t allow null values.	Foreign key can have multiple null values.
A table can have a single primary key.	We can have multiple foreign keys in a table that can reference different tables.
Primary Key can’t be duplicate. It means same value cannot be entered in the primary key column.	Foreign key can be duplicated. It means same value can be entered in foreign key column.
By default primary key implicitly creates a clustered index on the column and data in the table is physically organized in the sequence of clustered index.	Foreign key do not automatically create an index (clustered or non-clustered) but we can explicitly create an index on foreign key column.
We can define primary key constraint on temporary table	We can’t define foreign key constraint on temporary table
We can define primary key constraint on table variable	We can't define foreign key constraint on table variable
We can insert a value in primary key column that may or may not be present in child table containing the foreign key.	We can't insert a value in foreign key column that is not present in the primary key column in the referenced parent table.
We can't delete primary key value from the parent table which is used as a foreign key in child table. To delete we first need to delete that primary key value from the child table.	We can delete the foreign key value from the child table even though that refers to the primary key of the parent table.

 

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