Difference Between Primary Key And Unique key in RDMS

1) Unique key can be null but primary key cant be null.

2) Primary key can be referenced to other table as Foreign Key.

3) we can have multiple unique key in a table but PK is one and only
one.

4) Primary key in itself is unique key.

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

A primary key is a special constraint on a column or set of columns.
A primary key constraint ensures that the column(s) so designated have
no NULL values, and that every value is unique. Physically, a primary
key is implemented by the database system using a unique index, and
all the columns in the primary key must have been declared NOT NULL. A
table may have only one primary key, but it may be composite (consist
of more than one column).

A surrogate key is any column or set of columns that can be declared
as the primary key instead of a "real" or natural key. Sometimes there
can be several natural keys that could be declared as the primary key,
and these are all called candidate keys. So a surrogate is a candidate
key. A table could actually have more than one surrogate key, although
this would be unusual. The most common type of surrogate key is an
incrementing integer, such as an auto_increment column in MySQL, or a
sequence in Oracle, or an identity column in SQL Server.