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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:
Evaluating comparisons 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.
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