Designing primary key constraints
Each table can have one primary key. A primary key is a column or combination of columns that has the same properties as a unique constraint. You can use a primary key and foreign key constraints to define relationships between tables.
About this task
Because the primary key is used to identify a row in a table, it should be unique and have very few additions or deletions. A table cannot have more than one primary key, but it can have multiple unique keys. Primary keys are optional, and can be defined when a table is created or altered, using the PRIMARY KEY clause. They are also beneficial, because they order the data when data is exported or reorganized.
Primary key constraints
are designed like unique constraints, as described in Designing unique constraints.
The only difference is that you can have only one primary key constraint per
table, whereas, you can have many unique constraints.
Note: You can have
primary key constraints based on composite primary keys.