A null value is a special value that indicates the absence
of a value. Any column in a table or parameter or local variable
in a procedure, regardless of its data type, can contain null values
unless you specify NOT NULL for the column when you create the table
or the procedure. NULL is used as a placeholder for a value that
is missing or unknown. These properties of null values affect operations
on rows or parameters or local variables containing the following
values:
Null values always sort highest in
a sequence of values.
Two null values are not equal to each other except
in a GROUP BY or SELECT DISTINCT operation, or in a unique index.
An expression containing a null value evaluates
to null; for example, five minus null evaluates to null.
Because of these properties, ALLBASE/SQL ignores columns or
rows or parameters or local variables containing null values in
these situations:
Joining tables, if the join is on a column containing
null values
Executing aggregate functions
Evaluating if/while conditions or assignment expressions
In several SQL predicates, described in Chapter 9 “Search Conditions” you
can explicitly test for null values. In an application program,
you can use indicator variables to handle input and output null
values.
