Each datatype contains a distinguished empty value,
written null for its correspondence with null values in
relational databases.
The null value encodes a missing value.
It cannot be compared to any value,
including itself.
Thus,
the expressions
(null = null) and (null != null) are
both 0 (false).
Most built-in functions return null when given null.
For instance,
sqrt(null) returns null.
Since you cannot compare values to null,
there are special
SPLASH functions for handling null values.
The function
isnull returns 1 (true) if its argument is null,
and 0 (false) otherwise.
Because it is common to want to
choose a non-null value among a sequence of values,
there is
another function,
firstnonnull,
whose return
value is the first non-null value in the sequence of values.
The following example
returns 3:
firstnonnull(null,3,4,5)