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 the "Search Condition" chapter,
you can explicitly test for null values.
In an application program, you can use indicator variables
to handle input and output null values.
