Q:

What’s the difference between a primary key and a clustered index?

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Performance difference between Primary Key and Unique Clustered Index in SQL Server 

I make sure that I searched this forum but nobody asked this question before and I couldn't find any answer in anywhere too.
My question is = "What’s the difference between a primary key and a clustered index?"

A:

Well, for starters, one is a key, and the other one is an index.
In most database lingo, key is something that somehow identifies the data, with no explicit relation to the storage or performance of the data. And a primary key is a piece of data that uniquely identifies that data.
An index on the other hand is something that describes a (faster) way to access data. It does not (generally) concern itself with the integrity and meaning of the data, it's just concerned with performance and storage. In SQL Server specifically, a clustered index is an index that dictates the physical order of storage of the rows. The things that it does are quite complex, but a useful approximation is that the rows are ordered by the value of the clustered index. This means that when you do not specify a order clause, the data is likely to be sorted by the value of the clustered index.
So, they are completely different things, that kinda-sorta compliment each other. That is why SQL Server, when you create a primary key via the designer, throws in a free clustered index along with it.

A:

Before you can ask the difference between primary key and clustered index, you have to know that a key and an index are not the same thing.
A key can be a primary key or a foreign key. There can be only one primary key per table (but it might be more than one column). A key is a logical thing, it serves the business logic and defines the integrity of data. A foreign key is a reference to a primary key of another table. 
Indexes helps to speed up your queries, because it builds references to columns of your choice. So it creates separate files that helps your queries that use indexed columns. 
A clustered index is a special index that defines the physical order of your table (it should be a sequential data). 
I tried to explain this with my own words, but you'll find all resources you need with a google search (and I definitely recommend that you read a lot of this ! )

A:

Primary key is unique identifier for record. It's responsible for unique value of this field. It's simply existing or specially created field or group of fields that uniquely identifies row.
And clustered index is data structure that improves speed of data retrieval operations through an access of ordered records. Index is copy of one part of table. It takes additional physical place on hard disk.

