Outer Joins Introduce Nulls

The outer joins are meant to preserve rows from our reference relation and add to it columns from the outer relation when the join condition is satisfied. When the join condition is not satisfied, the outer joins then fill the columns from the outer relation with null values.

The outer join clause and null values

A typical example would be with calendar dates when we have not yet registered data at given dates. In our “motor racing” database example, we can ask for the name of the pole position’s driver and the final position. As the model registers the races early, some of them won’t have run yet, and so the results are not available in the database: