Processing null values

You do not need to know all the details of how CURSA manipulates null values internally. However, the following points may be useful.

• When a value for a new column is computed from an algebraic expression in which one of the input fields is null, then the result is also null. For example, if the new column was being computed for the expression `x + y' then the result would be null if either x or y (or both) were null. The generation of a null in this fashion is not considered an error and no message or warning is reported.

• When a relational expression is being used to generate a selection then rows for fields with null values occurring in the expression do not satisfy the expression. For example, if the selection was defined by `x > 2.0' then rows where x is null would not satisfy the expression.

• The function NULL is available to determine whether a field is null or not. For example, to select the rows for which column x is not null the following expression would be used: `.NOT. NULL(x)'.