An AutoNumber field can be set to automatically enter a sequential number as each record is added to the table. Designating such a field as the primary key for a table is the simplest way to create a primary key. If you don't set a primary key before saving a newly created table, Microsoft Access will ask if you want it to create a primary key for you. If you answer Yes, Microsoft Access will create an AutoNumber primary key.
AutoNumber primary keys in a replicated database
There are additional considerations if your table will be used with database replication.
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If fewer than 100 records are routinely added between synchronizing replicas, use a Long Integer setting for the FieldSize property to take up less disk space.
If more than 100 records are routinely added between synchronizing replicas, you should use Replication ID for the FieldSize property setting to prevent records from being assigned the same primary key value in each replica. Note, however, that an AutoNumber field with a Replication ID field size produces a 128-bit value that will require more disk space.
If you have a field that contains unique values such as ID numbers or part numbers, you can designate that field as the primary key. You can specify a primary key for a field that already contains data as long as that field does not contain duplicate values or Null values.
In situations where you can't guarantee the uniqueness of any single field, you may be able to designate two or more fields as the primary key. The most common situation where this arises is in the table used to relate two other tables in a many-to-many relationship. For example, an Order Details table can relate the Orders and Products tables. Its primary key consists of two fields: OrderID and ProductID. The Order Details table can list many products and many orders, but each product can only be listed once per order, so combining the OrderID and ProductID fields produces an appropriate primary key.
Each product can be listed only once per order.
Another example would be an inventory database that uses a field part number of two or more fields (part and subpart).
If you are in doubt about whether you can select an appropriate combination of fields for a multiple-field primary key, you should probably add an AutoNumber field and designate it as the primary key instead. For example, combining FirstName and LastName fields to produce a primary key is not a good choice, since you may eventually encounter duplication in the combination of these two fields.
In a multiple-field primary key, field order may be important to you. The fields in a multiple-field primary key are sorted according to their order in table Design view. You can change the order of the primary key fields in the Indexes window.