What is the difference between Primary Key and Unique Key

• In a table there can be only one column on which we implement Primary Key Constraint while we can implement Unique Key Constraints on more than one column.
• There can be no NULL value in column having Primary Key Constraint while NULL value can be allow in column having Unique Key Constraint.
• In column having Primary Key Constraint Cluster Index created by default while in column having Unique Key Constraint NonCluster Index creates by default.