Difference between SQLServer primary key and unique constraint, sqlserver primary key constraint

Difference between SQLServer primary key and unique constraint, sqlserver primary key constraint

First of all, the primary key is also called a primary key constraint. It is also a constraint. Let's look at the creation Syntax of 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 that the field values be unique. In addition, they have the following differences:

· One table can have only one primary key, but multiple unique constraints;
· The primary key field value cannot be NULL. The unique constraint field value can be NULL;
· The 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;
· SQLServer creates clustered indexes for primary key fields by default and non-clustered indexes for unique constraint fields;

Primary Key, unique, but not empty; unique, unique, but can be empty