The following table summarizes the differences between keys and indexes, using the primary key and primary index for purposes of comparison.
| Primary Key | Primary Index |
|---|---|
| Important element of logical data model. | Not used in logical data model. |
| Used to maintain referential integrity. | Used to distribute and retrieve data. |
| Must be unique to identify each row. | Can be unique or nonunique. |
| Values cannot change. | Values can change. |
| Cannot be null. | Can be null. |
| Does not imply access path. | Defines the most common access path. |
| Not required for physical table definition. | Required for physical table definition. |