Returns 1 if the query is active, positioned on a valid record and the
NULL; otherwise returns 0.
Reports whether the current field pointed at by an active query positioned on
a valid record is
qid (scalar) – query number.
field (scalar) – index into result set.
ret (scalar) – 1 if the field is
NULLor 0 otherwise.
Note that for some drivers,
dbQueryIsNull() will not return accurate
information until after an attempt is made to retrieve data.