What is the difference between a primary key and a unique constraint in SQLServer

What is the difference between primary key and unique constraint in SQLServer? I believe many inexperienced people don't know what to do about it. Therefore, this article summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

First of all, let's make it clear that the primary key, also known as the primary key constraint, is also a constraint. Take a look at the syntax for creating it and the unique constraint:

Alter table Person add constraint PK_Id primary key (Id) alter table Person add constraint UQ_Name unique (Name)

Both primary keys and unique constraints require field values to be unique, and in addition, they have the following differences:

A table can only have one primary key, but can have multiple unique constraints; the primary key field value cannot be NULL, the unique constraint field value can be NULL; primary key field can be used as the foreign key of other tables, and the unique constraint field cannot be used as the foreign key of other tables; SQL Server defaults to create a clustered index for the primary key field and a non-clustered index for the unique constraint field

Primary key, unique, but cannot be empty; unique constraint, unique, but can be empty

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