The IS DISTINCT FROM and IS NOT DISTINCT FROM search conditions are sargable and evaluate to TRUE or FALSE. See Using predicates in queries.
The IS NOT DISTINCT FROM search condition evaluates to TRUE if expression1 is equal to expression2, or if both expressions are NULL. This is equivalent to a combination of two search conditions, as follows:
The IS DISTINCT FROM syntax reverses the meaning. That is, IS DISTINCT FROM evaluates to TRUE if expression1 is not equal to expression2, and at least one of the expressions is not NULL. This is equivalent to the following:
SQL/2008
The IS [NOT] DISTINCT FROM predicate is defined in SQL/2008 standard. The IS DISTINCT FROM predicate is Feature T151,
"DISTINCT predicate", of the SQL/2008 standard. The IS NOT DISTINCT FROM predicate is Feature T152, "DISTINCT predicate with
negation", of the SQL/2008 standard.
Syntax