Three-valued Logic¶

Starting with Denodo 5.5, Virtual DataPort is fully conformant with the three-valued logic defined in the SQL standard. Previous versions of the Denodo Platform are not conformant with this standard. This change may lead to queries returning different results.

The three-valued logic defines that the data type boolean comprises the values True, False and Unknown, which is represented as the NULL value. Any comparison involving the null value or an Unknown truth will return an Unknown result.

The following example explains how this change can lead to different results. Let us say that we execute a query like this one:

SELECT ... WHERE NOT (a = b)

In one of the rows of the result, a is NULL and b is 1.

In previous versions of Virtual DataPort, (a = b) evaluates to false and NOT(a = b) to true. Therefore, this row is added to the result.

Virtual DataPort, as stated by the SQL standard, evaluates (a = b) to Unknown because a is NULL. The result of evaluating NOT (Unknown) is also Unknown. Therefore, this row is not added to the result.