Difference between primary key and unique key - MySQL Developer Tutorial

Below are the differences between Primary Key and Unique Key

1. Primary Key does not accept Null values but Unique Key column can accept Null values.
2. There is only One Primary Key per table but You can have multiple Unique Keys per Table
3. By default it added cluster index when you create Primary key, for Unique Key it creates Non-cluster index.

In below example, you can see that we created Primary key on idcustomer column and Unique Constraint on firstname and lastname.

CREATE TABLE customer (
  `idcustomer` int(11) NOT NULL,
  `firstname` varchar(50)  NULL,
  `lastname` varchar(30)  NULL,
  `age` int(11)  NOT NULL ,
  `phonenumber` char(11)  NULL,
  `dob` date DEFAULT NULL,
  `gender` char(1) NOT NULL
  ,Constraint pk_idcustomer Primary key(idcustomer),
  Constraint uq_firstname Unique Key(firstname),
  Constraint uq_lastname Unique Key(lastname)
  );

Check out detail demo for Difference between Primary Key and Unique Key in MySQL