On Wed, Oct 19, 2005 at 11:04:36PM -0700, CSN wrote: > So, does NULL != 'abc' always evaluate to false? It never evaluates to false -- it evaluates to NULL. http://www.postgresql.org/docs/8.0/interactive/functions-comparison.html The ordinary comparison operators yield null (signifying "unknown") when either input is null. Another way to do comparisons is with the IS DISTINCT FROM construct: expression IS DISTINCT FROM expression For non-null inputs this is the same as the <> operator. However, when both inputs are null it will return false, and when just one input is null it will return true. Thus it effectively acts as though null were a normal data value, rather than "unknown". Examples: test=> SELECT NULL = 'abc'; ?column? ---------- (1 row) test=> SELECT NULL <> 'abc'; ?column? ---------- (1 row) test=> SELECT NULL IS DISTINCT FROM 'abc'; ?column? ---------- t (1 row) test=> SELECT NULL IS DISTINCT FROM NULL; ?column? ---------- f (1 row) -- Michael Fuhr ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
