Nulls and the Outer Join - Teradata Database

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Nulls and the Outer Join

The outer join is a join that retains rows from one or both of the joined tables, depending on whether it is declared to be a left, right, or full outer join, respectively. This means that rows that do not match the rows produced by the inner join are extended in the outer join result by reporting nulls in the nonmatching columns. From the perspective of set theory, these nonmatching column values are not missing values, but the empty set, which is a real value. Because SQL represents the empty set as a null, the semantics of outer joins must be evaluated carefully whenever there are actual missing values in the inner join portion of the operation.

Ambiguities presented by nulls in the outer join also make query optimization more difficult, because certain key methods of query optimization such as transitive closure often cannot be achieved.