ALLBASE/SQL Reference Manual (36216-90216)

Chapter 7 215
Data Types
Null Values
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 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.