Unique Property

Applies To

Index Object.

Description

Sets or returns a value that indicates whether an Index object represents a unique (key) index for a table. For an Index object, this property setting is read/write until the object is appended to a collection, after which it's read-only.

Settings and Return Values

The setting or return value is a Boolean expression that specifies a unique index. The data type is Boolean. True (-1) indicates that the Index object represents a unique index.

Remarks

A unique index consists of one or more fields that logically arrange all records in a table in a unique, predefined order. If the index consists of one field, values in that field must be unique for the entire table. If the index consists of more than one field, each field can contain duplicate values, but each combination of values from all the indexed fields must be unique.

If both the Unique and Primary properties of an Index object are set to True, the index is unique and primary: It uniquely identifies all records in the table in a predefined, logical order. If the Primary property is set to False (0), the index is a secondary index. Secondary indexes (both key and nonkey) logically arrange records in a predefined order without serving as an identifier for records in the table.

Tips

You don't have to create indexes for tables, but in large, unindexed tables, accessing a specific record can take a long time.
Records read from tables without indexes are returned in no particular sequence.
The Attributes property of each Field object in the Index object determines the order of records and consequently determines the access techniques to use for that Index object.
A unique index helps optimize finding records.
Indexes don't affect the physical order of a base table — indexes affect only how the records are accessed by the table-type Recordset object when a particular index is chosen or when the Microsoft Jet database engine creates Recordset objects.