A primary key constraint ensures no duplicate values are entered in particular columns and that NULL values are not entered in those columns. You can use primary key constraints to enforce uniqueness as well as referential integrity. For example, the au_id column uniquely identifies each author stored in the authors table.
You create primary key constraints directly in a database diagram.
Note Some databases have different functionality for primary key constraints. For more information about the differences between databases, see Database Server Considerations. Consult your database documentation for details about how your database works with primary key constraints.
| To | See |
| Enforce uniqueness for values entered in specified columns | Defining a Primary Key |
| Change the column order, index name, clustered option, or fill factor | Modifying a Primary Key |
| Copy column properties from a primary key column to a foreign key column to relate the two columns | Copying Column Properties to a Foreign Key Column |
| Remove the requirement for uniqueness for the values entered in a column | Deleting a Primary Key Constraint |