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.

Null Values

Technical documentation

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