What is the difference between a <b>unique</b> key

What is the difference between a unique key and primary key?

Explanation

The PRIMARY KEY constraint is used to identify each row of data in a table. It doesn't allow NULL values. A table can have only one primary key, which may consist of single or multiple fields.
The UNIQUE KEY constraint ensures that all values in a column are different. It allows only one record to have a NULL value.
Both UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint though.
Several UNIQUE KEY constraints may exist per table, but only one PRIMARY KEY, though.

Theory

SQL UNIQUE Constraint
The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table. Read more: SQL UNIQUE Constraint
SQL PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys must contain UNIQUE values.
A primary key column cannot contain NULL values.
Most tables should have a primary key, and each table can have only ONE primary key.
Read more: SQL UNIQUE Constraint