Difference between Unique Key and Primary Keys

Question

I came across the following SQL in a book:

CREATE TABLE \'categories\'(
id SMALLINT NOT NULL AUTO INCREMENT,
category VARCHAR(30) NOT NULL,
PRIMARY KEY(\'id\         


        

Answer 1

A UNIQUE constraint and PRIMARY key both are similar and it provide unique enforce uniqueness of the column on which they are defined. Some are basic differences between Primary Key and Unique key are as follows.

Primary key

Primary key cannot have a NULL value. Each table can have only single primary key. Primary key is implemented as indexes on the table. By default this index is clustered index. Primary key can be related with another table's as a Foreign Key. We can generate ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.

Unique Constraint

Unique Constraint may have a NULL value. Each table can have more than one Unique Constraint. Unique Constraint is also implemented as indexes on the table. By default this index is Non-clustered index. Unique Constraint cannot be related with another table's as a Foreign Key. Unique Constraint doesn't support Auto Increment value.

You can find detailed information from: http://www.oracleinformation.com/2014/04/difference-between-primary-key-and-unique-key.html