What is difference between primary key and unique key?

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Primary key does not allow a null value but unique key does allow for one null value

Primary key cannot be referenced to another table as FK.

we can have multiple primary keys in a table but unique is one and only one

Primary key will create a clustered index by default

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Differences between Primary Key and Unique Key:

Primary Key:

A primary key cannot allow null values. (You cannot define a primary key on columns that allow nulls).
Each table can have at most one primary key.
On some RDBMS a primary key automatically generates a clustered table index by default.

Unique Key:

A unique key can allow null values. (You can define a unique key on columns that allow nulls).
Each table can have multiple unique keys.
On some RDBMS A unique key automatically generates a non-clustered table index by default.