Practical Programming, 3rd Edition

Keys and Constraints

Our query in the previous section relied on the fact that our regions and countries were uniquely identified by their names. A column in a table that is used this way is called a key. Ideally, a key’s values should be unique, just like the keys in a dictionary. We can tell the database to enforce this constraint by adding a PRIMARY KEY clause when we create the table. For example, when we created the PopByRegion table, we should have specified the primary key:

>>> cur.execute('''CREATE TABLE PopByRegion (
                   Region TEXT NOT NULL,
                   Population INTEGER NOT NULL,
                   PRIMARY KEY (Region))''')

Just as a key in a dictionary can be made up of multiple values, the primary key for a database table can consist ...