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About primary keys
Each table should include a field or set of fields that uniquely identifies
each record stored in the table. This information is called the primary
key and enforces the entity integrity of the table. A table can have only
one primary key constraint, and a column that participates in the primary
key constraint cannot accept null values.
Because primary key constraints ensure unique data, they are often defined
for identity column. 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 a primary key constraint is defined on more than one column, values
may be duplicated within one column, but each combination of values from
all the columns in the primary key constraint definition must be unique.