Difference between PRIMARY KEY and UNIQUE KEY

PRIMARY KEY	UNIQUE KEY
NULL value is not allowed	NULL is allowed, subject to the condition that only a single NULL is allowed. Inserting more null throws “Cannot insert duplicate key” error.
Table can have only one PRIMARY KEY constraint defined	Table can have multiple UNIQUE KEY constraints Defined
By default, if a clustered index is not defined already on the table, defining a PRIMARY KEY constraint results in a clustered index with same name as PRIMARY KEY	By default non-clustered index is create when a UNIQUE KEY constraint is added.
Purpose is to provide a unique identity to each row	Purpose is to avoid duplication