What is the difference between a unique index and a primary key index?

• Unique Index:
• Ensures that the values in the indexed columns are unique.
• Can be created on any column and allows NULL values (in most DBMS).
• A table can have multiple unique indexes.
• Primary Key Index:
• Enforces the uniqueness of the column(s) and also guarantees NOT NULL

constraint on the column(s).

• A table can only have one primary key.
• Automatically creates a unique index.

In short, both enforce uniqueness, but the primary key also guarantees that the column(s)

cannot be NULL.