HP 3000 Manuals HP 3000 Manuals
Null Values
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 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.