What is the difference between an index and a unique index?

What is indexes difference between unique and non unique indexes?

In addition to enforcing the uniqueness of data values, a unique index can also be used to improve data retrieval performance during query processing. Non-unique indexes are not used to enforce constraints on the tables with which they are associated.

Is index the same as unique constraint?

Conclusion. Generally there is no functional difference between a unique index and a unique constraint. The latter is also listed as a constraint, however this is only a method to emphasize the purpose of the index.

Is unique index faster than index?

In theory there is a slight difference in update performance as the engine needs to enforce uniqueness in a unique index, but in reality this is one going to be at most a few CPU cycles per row difference so will be unnoticeable. Only create a unique index for values that you know should be unique.

What is the difference between unique index and primary key?

You can only have one primary key per table, but multiple unique keys. Similarly, a primary key column doesn't accept null values, while unique key columns can contain one null value each. And finally, the primary key column has a unique clustered index while a unique key column has a unique non-clustered index.