A table can have only one clustered index because the clustered index is the physical order in which the records are stored in the database. You should not place a clustered index on a column containing volatile data because the records must be reordered every time you change the information in the column. Because an index is not a permanent part of the database, you can add a clustered index to a table and then remove it if the index is not appropriate. The following table lists table columns appropriate for clustered indexes and describes the benefits the clustered indexes provide.
| Table | Column | Benefit |
|---|---|---|
| Activity | GroupId | Improves retrieval of activities for a group. |
| ActivityType | ActivityTypeId | Provides fast single-row look-up. |
| Grade | GradeScaleId | Groups grades by grade scale, which is how the PT application implements grades. |
| GradeScale | GradeScaleId | Provides fast single-row look-up. |
| Group | GroupId | Provides fast single-row look-up. |
| Location | LocationId | Provides fast single-row look-up. |
| Logon | UserName | Provides fast single-row look-up. |
| Person | Provides fast single-row look-up when a user logs on to the PT application. | |
| PersonActivity | PersonId | Provides a way to sequence records by person. |
| PersonGroup | No candidate | N/A |