Both PRIMARY KEY and UNIQUE KEY enforces the Uniqueness of the values (i.e. avoids duplicate values) on the column[s] on which it is defined. Also these key’s can Uniquely identify each row in database table.
Primary Key:
- Can be only one in a table
- It never allows null values
- Primary Key is a unique key identifier and can not be null
- By default it adds a clustered index
- A table can have only one PRIMARY KEY Column[s]
- We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
- Primary key can be related with another table’s as a Foreign Key.
Unique Key:
- Can be more than one unique key in one table
- Unique key can have null values(only single null is allowed)
- It can be a candidate key
- Unique key can be null But only one Null value.
- By default it adds a UNIQUE non-clustered index
- A table can have more than one UNIQUE Key Column[s]
- Unique Constraint doesn’t supports Auto Increment value.
- Unique Constraint can not be related with another table’s as a Foreign Key.
