Unique Index Design Guidelines
A unique index guarantees that the index key contains no duplicate values and therefore every row in the table is in some way unique. Specifying a unique index makes sense only when uniqueness is a characteristic of the data itself. For example, if you want to make sure that the values in the NationalIDNumber column in the HumanResources.Employee table are unique, when the primary key is EmployeeID, create a UNIQUE constraint on the NationalIDNumbercolumn. If the user tries to enter the same value in that column for more than one employee, an error message is displayed and the duplicate value is not entered.
With multicolumn unique indexes, the index guarantees that each combination of values in the index key is unique. For example, if a unique index is created on a combination of LastName, FirstName, and MiddleName columns, no two rows in the table could have the same combination of values for these columns.
Both clustered and nonclustered indexes can be unique. Provided that the data in the column is unique, you can create both a unique clustered index and multiple unique nonclustered indexes on the same table.
The benefits of unique indexes include the following:
Data integrity of the defined columns is ensured.
Additional information helpful to the query optimizer is provided.
Creating a PRIMARY KEY or UNIQUE constraint automatically creates a unique index on the specified columns. There are no significant differences between creating a UNIQUE constraint and creating a unique index independent of a constraint. Data validation occurs in the same manner and the query optimizer does not differentiate between a unique index created by a constraint or manually created. However, you should create a UNIQUE or PRIMARY KEY constraint on the column when data integrity is the objective. By doing this the objective of the index will be clear.
A unique index, UNIQUE constraint, or PRIMARY KEY constraint cannot be created if duplicate key values exist in the data.
If the data is unique and you want uniqueness enforced, creating a unique index instead of a nonunique index on the same combination of columns provides additional information for the query optimizer that can produce more efficient execution plans. Creating a unique index (preferably by creating a UNIQUE constraint) is recommended in this case.
A unique nonclustered index can contain included nonkey columns. For more information, see Index with Included Columns.
There are several index options that can be specified when you create a unique index. You should give special consideration to following options:
For more information, see Setting Index Options.